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>
nah, brah, I prefer `VAR=${VAR:-value}` expansion. It's the "use default values" vs. your "assign default values" ;-)