chore: add mention of typos to CONTRIBUTING.md

Author: fsestini

CONTRIBUTING.md

@@ -99,7 +99,7 @@ See [the test documentation for more details](./tests/README.md).
 ### Linting
 
 Additionally, linting is enforced using `selene` to catch common errors, most of which are also caught by
-`lua-language-server`.
+`lua-language-server`. Source code spell checking is done via `typos`.
 
 ```sh
 make lint