docs(contributing): document conventional commit format

Author: Очеретович Оксана

CONTRIBUTING.md

@@ -32,3 +32,15 @@ If you're looking for a good place to start, look for issues labeled ["good firs
 - I have reviewed my diff in github, line by line and removed unexpected formatting changes, testing logs, or commented-out code.
 - Every change is related to the PR description.
 - I have [linked](https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue) this pull request to relevant issues (if any exist).
+
+## Commit messages
+
+This repository follows the Conventional Commits specification.
+
+Format:
+type(scope)!: short, lowercase description
+
+Examples:
+- fix(avm): correct calldata offset handling
+- chore(ci): avoid duplicate slack notifications
+- feat(bb)!: remove deprecated proving api