Add auto-changelog on Release

This PR adds a Github action which is triggered when a release is published. The action adds a new entry to the public Nexmo changelog with the contents of the release notes

Author: mheap

.github/main.workflow

@@ -0,0 +1,14 @@
+workflow "New release" {
+  on = "release"
+  resolves = ["Add Changelog"]
+}
+
+action "Add Changelog" {
+  uses = "nexmo/github-actions/nexmo-changelog@master"
+  secrets = ["CHANGELOG_AUTH_TOKEN"]
+  env = {
+    CHANGELOG_CATEGORY = "Server SDK"
+    CHANGELOG_SUBCATEGORY = "php"
+    CHANGELOG_RELEASE_TITLE = "nexmo-php"
+  }
+}