docs: Document how to use GH workflows in forks

.github/CONTRIBUTING.md

@@ -13,6 +13,7 @@
   * [Add code](#add-code)
   * [Finish with the documentation](#finish-with-the-documentation)
 * [Contributing to Python code](#contributing-to-python-code)
+* [Run tests in your fork](#run-tests-in-your-fork)
 
 ## Run and debug hooks locally
 
@@ -182,3 +183,11 @@ You can use [this PR](https://github.com/antonbabenko/pre-commit-terraform/pull/
     ```bash
     tox list
     ```
+
+## Run tests in your fork
+
+Go to your fork's `Actions` tab and click the big green button.
+
+![Enable workflows](/assets/contributing/enable_actions_in_fork.png)
+
+Now you can verify that the tests pass before submitting your PR.

.github/workflows/ci-cd.yml

@@ -84,6 +84,12 @@ jobs:
     name: ⚙️ Pre-set global build settings
 
     runs-on: ubuntu-latest
+    # Prevent run 'push' events for the branches in upstream repository as it
+    # already covered by 'pull_request' event
+    if: >-
+      github.repository != 'antonbabenko/pre-commit-terraform'
+      || github.event_name != 'push'
+      || github.ref == 'refs/heads/master'
 
     timeout-minutes: 1
 
@@ -499,7 +505,12 @@ jobs:
       codecov-token: ${{ secrets.CODECOV_TOKEN }}
 
   check:  # This job does nothing and is only used for the branch protection
-    if: always()
+    if: >-
+      always() && (
+        github.repository != 'antonbabenko/pre-commit-terraform'
+        || github.event_name != 'push'
+        || github.ref == 'refs/heads/master'
+      )
 
     needs:
     - lint