PR and issue templates and contributing guidelines

[jantman]

Create issue and PR templates. PR templates should link to the checklist, and ask if all tests have been run, including versioncheck if it was touched. Issue templates should include fields for the output (with --debug), the limitchecker version, the install method, the python version and OS/distro, and if installed with pip, the output of pip freeze from the venv.

Let's also add a CONTRIBUTING.md and just have it reference/link to the relevant RTD docs.

See https://github.com/blog/2111-issue-and-pull-request-templates and https://github.com/blog/1184-contributing-guidelines

[jantman]

CONTRIBUTING.md should just point to http://awslimitchecker.readthedocs.org/en/develop/development.html - it might as well go to develop, in case something's changed but not yet made it into master.

[jantman]

merged to develop in #140