Migrate Github Actions to YAML format

Author: mheap

.github/main.workflow

@@ -0,0 +1,16 @@
+name: Release published
+on:
+  release:
+    types: [published]
+jobs:
+  add-changelog:
+    name: Add Changelog
+    runs-on: ubuntu-latest
+    steps:
+    - name: Add Changelog
+      uses: nexmo/github-actions/nexmo-changelog@master
+      env:
+        CHANGELOG_AUTH_TOKEN: ${{ secrets.CHANGELOG_AUTH_TOKEN }}
+        CHANGELOG_CATEGORY: Server SDK
+        CHANGELOG_RELEASE_TITLE: nexmo-php
+        CHANGELOG_SUBCATEGORY: php