chore: add first contributing guidelines

CONTRIBUTING.md

@@ -0,0 +1,26 @@
+# Contributing
+
+[cargo-make]: https://github.com/sagiegurari/cargo-make "cargo-make"
+
+`tui` is an ordinary Rust project where common tasks are managed with [cargo-make]. It wraps common
+`cargo` commands with sane defaults depending on your platform of choice. Bulding the project should
+be as easy as running `cargo make`.
+
+## Continous Integration
+
+We use Github Actions for the CI where we perform the following checks:
+- The code should compile on `stable` and the Minimum Supported Rust Version (MSRV).
+- The tests (docs, lib, tests and examples) should pass.
+- The code should conform to the default format enforced by `rustfmt`.
+- The code should not contain common style issues `clippy`.
+
+You can also check most of those things yourself locally using `cargo make ci` which will offer you
+a shorter feedback loop.
+
+## Tests
+
+The test coverage of the crate is far from being ideal but we already have a fair amount of tests in
+place. Beside the usal doc and unit tests, one of the most valuable test you can write for `tui` is
+a test again the `TestBackend` which allows you to assert the content of the output buffer that
+would have been flushed to the termminal after a given draw call (see `widgets_block_renders` in
+[tests/widgets_block.rs](./tests/widget_block.rs) for an example).