Added localization instructions to CONTRIBUTING.md (#20451)

* Added localization instructions to CONTRIBUTING.md

Fixes #20418

* Corrected mention of generated ts file

* Mentioned coding guidelines

Author: Josh Goldberg

CONTRIBUTING.md

@@ -183,3 +183,10 @@ jake baseline-accept
 ```
 
 to establish the new baselines as the desired behavior. This will change the files in `tests\baselines\reference`, which should be included as part of your commit. It's important to carefully validate changes in the baselines.
+
+## Localization
+
+All strings the user may see are stored in [`diagnosticMessages.json`](./src/compiler/diagnosticMessages.json).
+If you make changes to it, run `jake generate-diagnostics` to push them to the `Diagnostic` interface in [`diagnosticInformationMap.generated.ts`](./src/compiler/diagnosticInformationMap.generated.ts).
+
+See [coding guidelines on diagnostic messages](https://github.com/Microsoft/TypeScript/wiki/Coding-guidelines#diagnostic-messages).