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>
like this:
`curl -s https://raw.githubusercontent.com/docker/docker/master/contrib/check-config.sh | bash`