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

CONTRIBUTING.md

@@ -64,10 +64,11 @@ packages which you can install with
 
 Make sure you
 
-  * Add documentation for a new feature (see below for where)
-  * Add unit tests for a new feature
+  * Add [documentation](#writing-documentation) for a new feature.
+  * Follow the [commit message guidelines](#commit-messages).
+  * Add [unit tests](#testing) for a new feature
   * squash commits down to one per feature
-  * rebase to master `git rebase master`
+  * rebase to master with `git rebase master`
 
 When you are done with that
 
@@ -203,14 +204,20 @@ file.
 ## Commit messages ##
 
 Please make the first line of your commit message a summary of the
-change, and prefix it with the directory of the change followed by a
-colon.  The changelog gets made by looking at just these first lines
-so make it good!
+change that a user (not a developer) of rclone would like to read, and
+prefix it with the directory of the change followed by a colon.  The
+changelog gets made by looking at just these first lines so make it
+good!
 
 If you have more to say about the commit, then enter a blank line and
 carry on the description.  Remember to say why the change was needed -
 the commit itself shows what was changed.
 
+Writing more is better than less.  Comparing the behaviour before the
+change to that after the change is very useful.  Imagine you are
+writing to yourself in 12 months time when you've forgotten everything
+about what you just did and you need to get up to speed quickly.
+
 If the change fixes an issue then write `Fixes #1234` in the commit
 message.  This can be on the subject line if it will fit.  If you
 don't want to close the associated issue just put `#1234` and the
@@ -258,9 +265,8 @@ To add a dependency `github.com/ncw/new_dependency` see the
 instructions below.  These will fetch the dependency, add it to
 `go.mod` and `go.sum` and vendor it for older go versions.
 
-    export GO111MODULE=on
-    go get github.com/ncw/new_dependency
-    go mod vendor
+    GO111MODULE=on go get github.com/ncw/new_dependency
+    GO111MODULE=on go mod vendor
 
 You can add constraints on that package when doing `go get` (see the
 go docs linked above), but don't unless you really need to.
@@ -275,9 +281,8 @@ in `vendor`.
 
 If you need to update a dependency then run
 
-    export GO111MODULE=on
-    go get -u github.com/pkg/errors
-    go mod vendor
+    GO111MODULE=on go get -u github.com/pkg/errors
+    GO111MODULE=on go mod vendor
 
 Check in in a single commit as above.