Merge the CONTRIBUTING.md files

docs/contributing-coding-guidelines.md

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+# Coding guidelines
+
+For the most part, Wasmtime and Cranelift follow common Rust conventions and
+[pull request] (PR) workflows, though we do have a few additional things to
+be aware of.
+
+[pull request]: https://help.github.com/articles/about-pull-requests/
+
+### rustfmt
+
+All PRs must be formatted according to rustfmt, and this is checked in the
+continuous integration tests. You can format code locally with:
+
+```sh
+$ cargo fmt
+```
+
+at the root of the repository. You can find [more information about rustfmt
+online](https://github.com/rust-lang/rustfmt) too, such as how to configure
+your editor.
+
+### Rustc version support
+
+Wasmtime supports the current stable version of Rust.
+
+Cranelift supports stable Rust, and follows the [Rust Update Policy for
+Firefox].
+
+Some of the developer scripts depend on nightly Rust, for example to run
+clippy and other tools, however we avoid depending on these for the main
+build.
+
+[Rust Update Policy for Firefox]: https://wiki.mozilla.org/Rust_Update_Policy_for_Firefox#Schedule