Add pull request template

I'm tierd of telling people to signoff their work:)

Signed-off-by: Felix Weilbach <felix.weilbach@nextcloud.com>

.github/pull_request_template.md

@@ -0,0 +1,14 @@
+<!-- 
+Thanks for opening a pull request on the Nextcloud desktop client.
+
+Instead of a Contributor License Agreement (CLA) we use a Developer Certificate of Origin (DCO).
+https://en.wikipedia.org/wiki/Developer_Certificate_of_Origin
+
+To accept that DCO, please make sure that you add a line like
+Signed-off-by: Random Developer <random@developer.example.org>
+at the end of each commit message.
+
+This Signed-off-by trailer can be added automatically by git if you pass --signoff or -s to git commit.
+See also:
+https://git-scm.com/docs/git-commit#Documentation/git-commit.txt---no-signoff
+-->