Add pull request template (#2942)

Provide contributors with a checklist to ensure quality PRs and to
avoid common pitfalls. The items in the checklist are mostly beyond
the capabilities of automated checker tools and more like reminders
of best practices. Still, they can save lot of valuable reviewer
time (hopefully).

JerryScript-DCO-1.0-Signed-off-by: Akos Kiss akiss@inf.u-szeged.hu

Author: akosthekiss

.github/pull_request_template.md

@@ -0,0 +1,19 @@
+**PLEASE REMOVE THIS TEMPLATE BEFORE SUBMITTING**
+
+Before submitting a PR, please, make sure that:
+
+- Changes are in a separate branch, not in master.
+- The branch contains only one commit on top of master (if not, squash them into
+  one commit).
+- The commit has a descriptive commit message with a concise title (first line).
+- The commit message contains `fixes #XXXX` or `closes #XXXX` to auto-close the
+  issue(s) that the PR fixes (if any).
+- Tests for the changes have been added (for bug fixes / features).
+- Documentation has been added / updated (if applicable).
+- All new and existing tests passed locally (if not, fix them first and amend
+  the commit).
+
+IMPORTANT: Please review the CONTRIBUTING.md file for detailed contributing
+guidelines.
+
+**PLEASE REMOVE THIS TEMPLATE BEFORE SUBMITTING**