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/ISSUE_TEMPLATE.md

@@ -0,0 +1,31 @@
+<!--
+
+Welcome :-) We understand you are having a problem with rclone; we want to help you with that!
+
+If you've just got a question or aren't sure if you've found a bug then please use the rclone forum:
+
+    https://forum.rclone.org/
+
+instead of filing an issue for a quick response.
+
+If you are reporting a bug or asking for a new feature then please use one of the templates here:
+
+    https://github.com/ncw/rclone/issues/new
+
+otherwise fill in the form below.
+
+Thank you
+
+The Rclone Developers
+
+-->
+
+
+#### Output of `rclone version`
+
+
+
+#### Describe the issue
+
+
+