Add publish workflow

Author: Flo0807

.github/workflows/ci.yml

@@ -2,6 +2,8 @@ name: CI
 
 on:
   push:
+  release:
+    types: [published]
 
 jobs:
   ci:
@@ -22,3 +24,9 @@ jobs:
 
       - name: Run Tests
         run: mix test
+
+      - name: Publish package
+        if: github.event_name == 'release'
+        env:
+          HEX_API_KEY: ${{ secrets.HEX_API_KEY }}
+        run: mix hex.publish --yes