This is an update to the PR template.
It mostly adds text and some checks related to how good the template
is filled in and whether the `develop` branch was selected as base.
It also updates the README and the bug_report template to reflect
the changes. Additionally, in the README some relevant shields from
shields.io are added.
The labels follow [label-schema.org][lsorg] and the [OpenContainer
image spec, section annotations][ocis], specifications.
[lsorg]: http://label-schema.org/rc1/
[ocis]: https://github.com/opencontainers/image-spec/blob/master/annotations.md
