doc: add README to embassy-macro

Documents the main and task macros.

embassy-macros/README.md

@@ -0,0 +1,21 @@
+# embassy-macros
+
+An [Embassy](https://embassy.dev) project.
+
+Macros for creating the main entry point and tasks that can be spawned by `embassy-executor`. 
+
+NOTE: The macros are re-exported by the `embassy-executor` crate which should be used instead of adding a direct dependency on the `embassy-macros` crate.
+
+## Minimum supported Rust version (MSRV)
+
+The `task` and `main` macros require the type alias impl trait (TAIT) nightly feature in order to compile.
+
+## License
+
+This work is licensed under either of
+
+- Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or
+  <http://www.apache.org/licenses/LICENSE-2.0>)
+- MIT license ([LICENSE-MIT](LICENSE-MIT) or <http://opensource.org/licenses/MIT>)
+
+at your option.