Update CONTRIBUTING.md to include grammar validation

Summary: Explicitly mention that the grammar is used as the front-end for ELP, and that any change must not break the ability to run `cargo xtask codegen` in ELP.

Reviewed By: perehonchuk

Differential Revision: D50004829

fbshipit-source-id: f9525423922ebec69c366594a936f0c573a01ec2

CONTRIBUTING.md

@@ -17,6 +17,33 @@ We actively welcome your pull requests.
 3. Ensure the test suite passes.
 4. If you haven't already, complete the Contributor License Agreement ("CLA").
 
+### Grammar Validation
+
+This grammar is used as the front end for the [Erlang Language Platform (ELP)](https://github.com/WhatsApp/erlang-language-platform).
+
+As part of this process, we generate an ELP AST structure from the generated `grammar.json` file.
+
+So a pull request will not be landed until it has matching changes in ELP.
+
+The most important thing is to check that the code generation does not
+fail, as it imposes additional constraints on the shape of the grammar.
+
+Thes can be checked by
+
+1. Setting up an ELP development environment according it [CONTRIBUTING](https://github.com/WhatsApp/erlang-language-platform/blob/main/CONTRIBUTING.md)
+
+2. Updating
+   [Cargo.toml](https://github.com/WhatsApp/erlang-language-platform/blob/c4d3602e08418432a020abb734723727a8374549/Cargo.toml#L94)
+   to get the `tree-sitter-erlang` crate from your checked out changed `tree-sitter-erlang` repository.
+
+3. In the `ELP` repository, running `cargo xtask codegen`.  If it succeeds
+   without errors, the grammar change is acceptable.
+
+4. For bonus points you can then `cargo build` ELP and deal with
+   changes induced by the grammar, but that is not necessary.
+   However, the change will not land until a matching change is made
+   to ELP by a contributor.
+
 ## Contributor License Agreement ("CLA")
 
 In order to accept your pull request, we need you to submit a CLA. You only need