Add markdown feature documents

docs/features/async-main.md

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+# Async Task Main
+## [dotnet/csharplang proposal](https://github.com/dotnet/csharplang/blob/master/proposals/async-main.md)
+
+## Technical Details
+
+* The compiler must recognize `Task` and `Task<int>` as valid entrypoint return types in addition to `void` and `int`.
+* The compiler must allow `async` to be placed on a main method that returns a `Task` or a `Task<T>` (but not void).
+* The compiler must generate a shim method `$EntrypointMain` that mimics the arguments of the user-defined main.
+  * `static async Task Main(...)` -> `static void $EntrypointMain(...)`
+  * `static async Task<int> Main(...)` -> `static int $EntrypointMain(...)`
+  * The parameters between the user-defined main and the generated main should match exactly.
+* The body of the generated main should be `return Main(args...).GetAwaiter().GetResult();`

docs/features/async-main.test.md

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+Main areas to test:
+* Signature acceptance / rejection
+* Method body creation
+* Correct entrypoint assignment