added drone config to update github mirror

.drone.yml

@@ -163,4 +163,17 @@ pipeline:
       {{repo.owner}}/{{repo.name}}: \[{{build.status}}] Build {{build.number}}
       {{commit.author}} pushed to {{commit.branch}} {{commit.sha}}: `{{commit.message}}`
       Build started at {{datetime build.started "2006-Jan-02T15:04:05Z" "GMT+2"}} finished at {{datetime build.finished "2006-Jan-02T15:04:05Z" "GMT+2"}}.
-
+  
+# Update the mirror on github
+  github_mirror:
+    image: appleboy/drone-git-push
+    branch: master
+    remote: git@github.com:go-vikunja/api.git
+    remote_name: github
+    local_branch: master
+    followtags: true
+    force: false
+    commit: false
+    when:
+      event: [ push, tag ]
+      branch: [ master ]