Add local CI instructions to CONTRIBUTING.md

CONTRIBUTING.md

@@ -1,6 +1,15 @@
-### Pull Requests
+## Pull Requests
 
-Running `cargo clippy` on the branch to be merged must show no errors.
+### Continuous Integration (CI)
+
+All pull requests must pass a CI check before being merged. You can check if CI will pass locally with
+```shell
+cargo fmt --all -- --check && cargo build --verbose && cargo clippy -- -D warnings && cargo test --verbose -- --skip sourcegen_ast --skip sourcegen_ast_nodes
+```
+
+### Clippy
+
+One of the CI items is `cargo clippy`.
 To handle a lot of errors at the command line can use (for unix-like OS) `cargo clippy --color always &| less -R`.