Update contributor's guide with how to contribute to documentation

[joshuacc]

This causes confusion. See #164.

[fogus]

It would also be lovely to have a step-by-step guide to contributing. Along the lines of:

• checkout
• make code change (_ style adherence)
• make doc change
• make test change
• run tests
• generate docs to ensure they look nice

Something along those lines.

-- http://blog.fogus.me

-- http://github.com/fogus

[joshuacc]

@fogus Yep. I'll see if I can put something together.

[joshuacc]

@fogus Updated the contributor's guide to be step by step as requested. Let me know if you'd like to see any changes.