Add welcome bot

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>

.github/config.yml

+firstPRMergeComment: >
+    Thanks for your first pull request and welcome to the club!
+    Feel free to keep them comming!
+
+    Most developers hang out on IRC. So join \#nextcloud-dev on Freenode for a chat!