Revamp issue and PR templates and CONTRIBUTING guide

Thanks to @fd0 of the restic project for a very useful blog post and
something to plagiarise :-)

https://restic.net/blog/2018-09-09/GitHub-issue-templates

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,29 @@
+<!--
+Thank you very much for contributing code or documentation to rclone! Please
+fill out the following questions to make it easier for us to review your
+changes.
+
+You do not need to check all the boxes below all at once, feel free to take
+your time and add more commits. If you're done and ready for review, please
+check the last box.
+-->
+
+#### What is the purpose of this change?
+
+<!--
+Describe the changes here
+-->
+
+#### Was the change discussed in an issue or in the forum before?
+
+<!--
+Link issues and relevant forum posts here.
+-->
+
+#### Checklist
+
+- [ ] I have read the [contribution guidelines](https://github.com/ncw/rclone/blob/master/CONTRIBUTING.md#submitting-a-pull-request).
+- [ ] I have added tests for all changes in this PR if appropriate.
+- [ ] I have added documentation for the changes if appropriate.
+- [ ] All commit messages are in [house style](https://github.com/ncw/rclone/blob/master/CONTRIBUTING.md#commit-messages).
+- [ ] I'm done, this Pull Request is ready for review :-)