Update documentation

[jakefhyde]

Lots of gaps regarding usage, developing, testing, publishing, templates, etc.

[git-ival]

I'd like to add a request to document the existence, default location and potential usage of the config.yaml that corral generates during corral config. It seems that by default this is located at ~/.corral/config.yaml. This file can be used to modify the values of shared variables across all corral packages.