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>
lol. I bought thisco.de and haven't used it yet.
`curl -v report.thisco.de | bash`

but seriously though. All needed is:
* a way to fetch the template
* the defaults to populate (and how)
* a redirection (like `xdg-open`) and/or provide the url for them to click
* URL encode the whole damn thing (e.g. https://github.com/docker/docker/issues/new?title=Report%3A%20foo%20bar&body=%0ADescription%3A%0Ait%20bars%20when%20i%20foo%0A%0A%60docker%20version%60%3A%0AClient%20version%3A%201.3.0%0AClient%20API%20version%3A%201.15%0AGo%20version%20%28client%29%3A%20go1.3.1%0AGit%20commit%20%28client%29%3A%20c78088f%2F1.3.0%0AOS%2FArch%20%28client%29%3A%20linux%2Famd64%0AServer%20version%3A%201.3.0%0AServer%20API%20version%3A%201.15%0AGo%20version%20%28server%29%3A%20go1.3.1%0AGit%20commit%20%28server%29%3A%20c78088f%2F1.3.0%0A )