feat(counter): "Updating" and "Summary" sections

Add a section walking through some simple updates to the Counter
contract, and a Summary section to walk through what we saw in this
chapter and build some anticipation for the next.

src/SUMMARY.md

@@ -9,6 +9,8 @@
   - [Final Code](./counter/final_code.md)
   - [Using RAEN](./counter/working_with_contract.md)
   - [RAEN Admin](./counter/admin.md)
+  - [Updating the contract](./counter/updating.md)
+  - [Summary](./counter/summary.md)
 
 - [Status Message](./status-message/intro.md)
   - [Defining the data](./status-message/simple/simple.md)

src/counter/summary.md

@@ -0,0 +1,9 @@
+# Counter Summary
+
+In this chapter we saw our first Rust code, a simple NEAR smart contract that implements a counter. Anyone can call `get_num` to see the current value, and anyone can sign in and call change methods to change the number.
+
+We saw how RAEN Admin makes it easy to interact with contracts built with RAEN. We dug deeper and saw how we can change the contract, re-deploy, and see that RAEN Admin will automatically show the latest version of the contract.
+
+We even investigated some more advanced Rust features, such as changing settings in `Cargo.toml` and implementing a custom `Default` rather than deriving it.
+
+Overall, this Counter was a good way to learn some Rust and NEAR basics, but pretty silly as a real contract. Next, let's take a look at an example that does something more useful, storing different information for every user who calls the contract.

src/counter/updating.md

@@ -0,0 +1,96 @@
+# Updating the contract
+
+[Earlier](./first_steps.md) we said that if an `i8` is currently `127` and you increment it again, it will _overflow_, becoming `-128`. Likewise, it can _underflow_ if currently `-128` and you decrement.
+
+Let's see if we can make `val` overflow! And then let's see how to prevent this.
+
+# Method 1: mash `increment` 128 times
+
+With the existing contract, you could use the RAEN Admin panel and mash the Submit button on the `increment` form 128 times.
+
+This could take a while, though! Let's see if we can improve the code to make it easier.
+
+# Method 2: new `default`
+
+If you're following along with your own contract, you can change the default value of `val` to start at `127` rather than `0`. This is a good way to learn about where the default of `0` currently comes from.
+
+If you get stuck while following along, you can see this method and the following all implemented on [this branch](https://github.com/raendev/examples/pull/4/files).
+
+Above `pub struct Counter`, change the `derive` macro by removing the `Default` trait:
+
+```diff
+-#[derive(Default, BorshDeserialize, BorshSerialize)]
++#[derive(BorshDeserialize, BorshSerialize)]
+```
+
+Now, below the closing bracket of `pub struct Counter`, you can implement `default` manually:
+
+```rust
+pub struct Counter {
+    val: i8,
+}
+
+impl Default for Counter {
+    fn default() -> Self {
+        Self { val: 127 }
+    }
+}
+```
+
+## If you're following along
+
+If you're following along with your own code, you can build and re-deploy to see the new default:
+
+```bash
+raen build --release -q | xargs -I {} near dev-deploy --wasmFile {}
+```
+
+(This [uses xargs to](https://stackoverflow.com/a/42631216/249801) pipe the output of `raen build` to the `wasmFile` argument of `near dev-deploy`.)
+
+If you've already incremented or decremented the number, though, you'll see that this made no change. Which is what you'd expect! It only changes the _default_. That is, the value that is shown if you didn't actually change it and store the new value.
+
+To see this default in action, you can delete the `neardev` folder and run the `dev-deploy` command again. This will create a brand new NEAR testnet account before deploying your contract.
+
+Paste this new account name into RAEN Admin, check `get_num`, and see the number you set as a default! Call `increment` (you'll need to sign in again, since this is a new contract.)
+
+## If you're not
+
+The contract deployed at [counter.raendev.testnet](https://raen.dev/admin/#/counter.raendev.testnet) already has a value for `val` saved to contract storage. It will not make use of a new default.
+
+Let's try a different way.
+
+# Method 3: new `set_num`
+
+See if you can write your own `set_num` method based on the code for `increment` and `decrement` in the contract. If you need a hint, [here's what it looks like](https://github.com/raendev/examples/pull/4/files).
+
+## If you're following along
+
+Save your file with new `set_num`, then build and re-deploy:
+
+```bash
+raen build --release -q | xargs -I {} near dev-deploy --wasmFile {}
+```
+
+Unlike Method 2, you don't need to create a new testnet account. Just refresh the RAEN Admin panel and see the `set_num` method appear under Change Methods. Go ahead and try it out!
+
+## If you're not
+
+The [reference code](https://github.com/raendev/examples/pull/4/files) has been deployed to [v2.counter.raendev.testnet](https://raen.dev/admin/#/v2.counter.raendev.testnet) for you to play with.
+
+Note that this code already fixed the overflow issue. If you want to see the overflow in action, you'll have to follow along with your own code.
+
+# Fixing that overflow
+
+Now you can call `increment` and see that the number goes from `127` to `-128`. Oops! How do we fix it?
+
+Open up the `Cargo.toml` file. [Cargo.toml](https://doc.rust-lang.org/cargo/guide/cargo-toml-vs-cargo-lock.html) is to Rust what `package.json` is to NodeJS. You'll see sections for `dependencies` and `dev-dependencies` and more generic information about this package/contract/crate.
+
+See those lines with `overflow-checks = false`? Change them to `overflow-checks = true`. Now rebuild and redeploy:
+
+```bash
+raen build --release -q | xargs -I {} near dev-deploy --wasmFile {}
+```
+
+You'll see that this takes longer to build than when you make changes to your contract itself. Rust needs to recompile all of your dependencies using the new setting.
+
+Refresh your admin panel, set the number back to `127`, and call `increment`. You'll see an error message. This means the contract panicked rather than allowing the integer overflow. It worked!