ci: add pr title validator (gnolang#669)

.github/workflows/lint-pr.yml

@@ -0,0 +1,17 @@
+name: "Lint PR"
+
+on:
+  pull_request_target:
+    types:
+      - opened
+      - edited
+      - synchronize
+
+jobs:
+  main:
+    name: Validate PR title
+    runs-on: ubuntu-latest
+    steps:
+      - uses: amannn/action-semantic-pull-request@v5
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

CONTRIBUTING.md

@@ -267,11 +267,35 @@ The gno project tends to use the [Conventional Commits](https://www.conventional
 commit that goes into the main code stream (currently, the `master` branch).
 
 Each PR is squashed and merged into the main code stream, which means PR _titles_ should adhere to the Conventional
-Commit standard, and be short and precise.
+Commit standard, and be short and precise. This is practically enforced using a
+[linter check](https://github.com/amannn/action-semantic-pull-request). The type
+of PRs that are allowed are the following:
+
+* **build**\
+  Changes that affect the build system or external dependencies.
+* **chore**\
+  Other changes that don't modify source or test files.
+* **ci**\
+  Changes to our CI configuration files (ie. GitHub Actions) and scripts.
+* **docs**\
+  Documentation only changes.
+* **feat**\
+  A new feature.
+* **fix**\
+  A bug fix.
+* **perf**\
+  A code change that improves performance.
+* **refactor**\
+  A code change that neither fixes a bug nor adds a feature.
+* **revert**\
+  Reverts a previous commit.
+* **style**\
+  Changes that do not affect the meaning of the code (white-space, formatting, missing semi-colons, etc).
+* **test**\
+  Adding missing tests or correcting existing tests.
 
 A general rule of thumb:
 
-- Use Conventional Commits for PR titles
 - Never favor rewriting history in PRs (rebases have very few exceptions, like implementation rewrites;
 see [A Word on Rebasing](#a-word-on-rebasing))
 - Tend to make separate commits for logically separate changes