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/Bug.md

@@ -0,0 +1,50 @@
+---
+name: Bug report
+about: Report a problem with rclone
+---
+
+<!--
+
+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 think you might have found a bug, please can you try to replicate it with the latest beta?
+
+    https://beta.rclone.org/
+    
+If you can still replicate it with the latest beta, then please fill in the info below which makes our lives much easier.  A log with -vv will make our day :-)
+
+Thank you
+
+The Rclone Developers
+
+-->
+
+#### What is the problem you are having with rclone?
+
+
+
+#### What is your rclone version (output from `rclone version`)
+
+
+
+#### Which OS you are using and how many bits (eg Windows 7, 64 bit)
+
+
+
+####  Which cloud storage system are you using? (eg Google Drive)
+
+
+
+#### The command you were trying to run (eg `rclone copy /tmp remote:tmp`)
+
+
+
+#### A log from the command with the `-vv` flag (eg output from `rclone -vv copy /tmp remote:tmp`)
+
+