Added pull request template. (#56)

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,8 @@
+[//]: # (Thanks for opening this pull request! Before you proceed please make sure that you've an issue that explains what this pull request will do.
+         Make sure that all your commits link this issue e.g "My commit \(bunq/sdk_java#<issue nr>\)".
+         If this pr is changing files that are located in "com.bunq.sdk.model.generated" then this PR will be closed as these files must/can only be changed on bunq's side.)
+
+## This PR closes/fixes the following issues:
+[//]: # (If for some reason your PR does not require a pull request you can just mark this box as checked.)
+ - Closes bunq/sdk_java#
+    - [ ] Tested