Add pull request template

.github/pull_request_template.md

@@ -0,0 +1,10 @@
+The Joda-Time project has been running for many years now, and the codebase is stable.
+Java SE 8 contains a new date and time library that is the successor to Joda-Time.
+As such Joda-Time is primarily in maintainence modeand few pull requests are likely to be merged.
+
+**As a general rule, most enhancement PRs will now be rejected.**
+
+To save wasted effort, it is recommended that an issue is raised first.
+The issue should clearly indicate that a PR is intended, and request approval for the work.
+
+If you still want to raise a PR, please delete this text!