Document usage for gitlab/bitbucket

[IvanSanchez]

With support for repos other than github merged in #13, it should be nice to add a few lines to the readme to explain that functionality

[Rich-Harris]

ah, whoops, sorry — didn't realise you'd sent this PR, I just updated the README. It actually doesn't support custom git servers, because I'm not sure if there's a standard way to generate the .tar.gz URL. Would be glad to be enlightened though!

[melniker]

@Rich-Harris, I was planning on working on non-github support before I saw the issues closed today, and my plan included a config json file that gave the characteristics of the servers. I was going to pre-populate it for github/gitlab/butbucket but it could then be easily extendable for other servers. It would specify every property you'd need, including URI format & location of the tarball.

This way, degit would be easily extendable not only for public git servers (which could be put into debit/master via PR) but also for private instances inside a big corporation, e.g.

Could also of course take these config settings via cli parameters.

Thoughts?

[Rich-Harris]

@melniker I like it! That sounds great