Add Feature Documentation (#154)

* Add basic information for creating a new feature

* Add instruction to add new command handler

* accept grammar correction suggestion

Co-authored-by: Frank Dana <[email protected]>

* Add example of where the TryFromState fns can be found

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Co-authored-by: albertotirla <[email protected]>
Co-authored-by: Frank Dana <[email protected]>

CONTRIBUTING.md

@@ -10,6 +10,28 @@ These can be added to your pre-commit hooks to automate the checks. Beyond these
 
 Besides this, we do not have any specific contribution guidelines or codes of conduct for now, however most likely these will be fleshed out as Odilia matures more.
 
+## How To Add A Feature
+
+Odilia has a unique architecture which embeds alot of code into types.
+Here is what you need to know if you want to add a new feature:
+
+- [ ] Find the AT-SPI event that corresponds to the desired feature
+    - Take a look at the docs for `common/events/` and try to find the right event.
+    - For example, to create a feature that reads out changed text in an aria-live region, you would want the event `TextChangedEvent`.
+- [ ] Decide if there are any prerequisites to the feature being triggered (think: focused window only, open tab only, etc.)
+- [ ] See if the prerequisite is already defined
+    - Check out the `odilia/src/tower/cache_event.rs` file for types that implement the `Predicate` trait (defined in the `refinement` create).
+- [ ] Decide what, if any, state is required for your feature (caret position, last focused item, etc.)
+    - For example: to know if the current window is focused, the current window must be stored in the state of the screen reader.
+- [ ] Check to see if we already have your state info as a type; you can check this at `odilia/src/state.rs`.
+		- If not, make a [newtype](https://doc.rust-lang.org/book/ch19-04-advanced-types.html#using-the-newtype-pattern-for-type-safety-and-abstraction), add it to the `State` struct, then implement the `TryFromState` trait for the type (so it can be extracted by the `TryFromState` implementation).
+		- An example of how to implement `TryFromState` can be found in `odilia/src/state.rs`
+- [ ] Implement a new async function that takes `PreRequisiteType<EventType>`, and `StateType` (if necessary). That function can return a list of `Command`s that Odilia will act on.
+    - Then, add it to the list of `.atspi_listener(fn_name)` calls in `main`.
+    - The list of possible `Command`s can be found in the type `OdiliaCommand` enum in `common/src/commands.rs`.
+- [ ] If a new `Command` is required, create a newtype, implement the `IntoCommands` trait, add it as a variant to the enum, then finally implement the `CommandType` trait.
+    - To add funcionality to this command, create an `async fn` that takes `Command(NewCommandType)` and `NewStateType` (if necessary). Finally, add it to the list of `.command_listener(fn_name)` calls in `main`.
+
 ## Performance Benchmarking
 
 If you'd like detailed performance benchmarks, we recommend using the `flamegraph` package to show performance bottlenecks.