Add cookbook section

Add a "cookbook" section that describes how to handle different
situations in source result files.

Address http://github.com/openelections/docs/issues/21
Merge pull request #24 from openelections/21-cookbook

Add cookbook section
Update repo URLs

Update the URLs to reflect the core repo URL change from
https://github.com/openelections/core/ to
https://github.com/openelections/openelections-core/.

Addresses https://github.com/openelections/openelections-core/issues/229