CONTRIBUTING: provide a template for new issues

_maybe_ we could even automate this to populate and complain in some
situations.

I see https://github.com/isaacs/github/issues/99 that is old and needs
some love from @github ...

@tianon @unclejack @shykes 

Signed-off-by: Vincent Batts <vbatts@redhat.com>
```sh
: ${DOCKER_ISSUE_URL:='https://github.com/docker/docker/issues/new'}
```
? :smile: