Add contributing subcommand

[kemitchell]

cc @kennymalac

[kennymalac]

Hey, I myself am not a Go programmer so I cannot comment on the code, though I will say it looks fine to me. I consider this a minor point: I noticed that the CONTRIBUTING file regex is hardcoded to write to that file, I was wondering if you would consider a flag for making the filename explicit (i.e. -filename blah.md). So for example I have a CONTRIBUTING.md file in Media Center that contains my Contributor's guide. I may call this ContributorsGuide.md or something to that effect; however, I do not know if all projects would necessarily want it appended into that file. Just a thought, I don't know if the other subcommands have that option to not hardcode the filename. I know for LICENSE it doesn't make much sense for that filename to be configurable, but perhaps here it does?

[kemitchell]

Here is the text that the subcommand appends to CONTRIBUTING:

Licensing

If you submit a pull request, please be prepared to license your contributions under the terms of the Charity Public License, a modern evolution of licenses like MIT and the two-clause BSD license.

The code doesn't always write to CONTRIBUTING.md. Rather it looks for a CONTRIBUTING file, upper or lower case, with an extension or not, and appends to that file.

[kennymalac]

Well yes, I meant the CONTRIBUTING/contributing filename itself being hardcoded, not the extension of that file. I suppose it's fine that way, do you know if any project would want to deviate from that?

[kemitchell]

@kennymalac I'm sure some may, but AFAIK, GitHub only "supports" CONTRIBUTING and variations in its UI.