Add an initial contributing doc.

Author: ehuss

CONTRIBUTING.md

@@ -0,0 +1,16 @@
+# Contributing to The Rust Specification
+
+> [!WARNING]
+> The Rust Specification is still in the early days of getting started and is
+> not yet ready for outside contributors. Please get in contact with the team
+> on [Zulip] if you are interested in participating.
+
+[Zulip]: https://rust-lang.zulipchat.com/#narrow/stream/399173-t-spec
+
+## Building
+
+See the [Building docs](README.md#building) for information on generating HTML from the specification source files.
+
+## Authoring guide
+
+See the [Authoring Guide](docs/authoring.md) for information about working with the specification source files.