Add a quick start guide to contributing

This is a rough port of my blog post at

Feel free to rip out the vanity link and change in any other way.

It's more a narrative than a proper guide, so may either need editing or need renaming. I'm happy to make changes or have someone else edit it.

NicoAlt on IRC told me to make a dir called _guides but I wasn't able to get it to work (even when editing _config.yml) so I've just made a new file in _pages for now.