add docs about github actions

Author: krzkaczor

supported-ci.md

@@ -2,13 +2,43 @@
 
 - Circle CI
 - Travis CI
+- Github Actions
 - BuildKite CI
 - Semaphore CI
 
 Is your favorite CI provider missing? Just create an issue
 [here](https://github.com/codechecks/monorepo). You can implement it on your own, read this
 [quick guide](adding-new-ci.md)
 
+## Github Actions
+
+1. Provide CC_SECRET value in your projects settings
+2. Pass it explicitly to the job.
+
+Example:
+
+```yml
+on: push
+name: Mainflow
+jobs:
+  build-and-codechecks:
+    name: build and run codechecks
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@master
+      - name: yarn
+        run: |
+          yarn install --frozen-lockfile
+
+          npx codechecks
+        env:
+          CC_SECRET: ${{ secrets.CC_SECRET }}
+```
+
+NOTE: It requires
+[speculative-branch execution](https://github.com/codechecks/docs/blob/master/configuration.md#speculative-branch-execution)
+feature. So please configure branch priority if you use other branches than `master`.
+
 ## Semaphore CI Known issues
 
 Note that passing secrets requires specifying