Update readme from code (#1188)

### What
Update readme using make target and add a CI job to check its updated.

### Why
Noticed our readme is out of date with the code it is generated from.

.github/workflows/rust.yml

@@ -17,7 +17,7 @@ jobs:
 
   complete:
     if: always()
-    needs: [fmt, check-git-rev-deps, build-and-test, docs]
+    needs: [fmt, check-git-rev-deps, build-and-test, docs, readme]
     runs-on: ubuntu-latest
     steps:
     - if: contains(needs.*.result, 'failure') || contains(needs.*.result, 'cancelled')
@@ -132,6 +132,15 @@ jobs:
     - run: rustup install nightly
     - run: make doc
 
+  readme:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v3
+    - uses: stellar/actions/rust-cache@main
+    - run: rustup install nightly
+    - run: make readme
+    - run: git add -N . && git diff HEAD --exit-code
+
   publish-dry-run:
     if: github.event_name == 'push' || startsWith(github.head_ref, 'release/')
     strategy:

soroban-sdk/README.md

@@ -8,8 +8,9 @@ See [soroban.stellar.org](https://soroban.stellar.org) for documentation.
 ### Examples
 
 ```rust
-use soroban_sdk::{contractimpl, vec, BytesN, Env, Symbol, Vec};
+use soroban_sdk::{contract, contractimpl, vec, symbol_short, BytesN, Env, Symbol, Vec};
 
+#[contract]
 pub struct HelloContract;
 
 #[contractimpl]
@@ -21,6 +22,9 @@ impl HelloContract {
 
 #[test]
 fn test() {
+# }
+# #[cfg(feature = "testutils")]
+# fn main() {
     let env = Env::default();
     let contract_id = env.register_contract(None, HelloContract);
     let client = HelloContractClient::new(&env, &contract_id);
@@ -29,6 +33,9 @@ fn test() {
 
     assert_eq!(words, vec![&env, symbol_short!("Hello"), symbol_short!("Dev"),]);
 }
+# #[cfg(not(feature = "testutils"))]
+# fn main() { }
 ```
 
-More examples are available at <https://soroban.stellar.org/docs/category/how-to-guides>.
+More examples are available at <https://soroban.stellar.org/docs/category/basic-tutorials>
+and <https://soroban.stellar.org/docs/category/advanced-tutorials>.