Add documentation for the `commitish` option

[Cyberboss]

Closes #83

[damntrecky]

Was great that someone did this. I would suggest linking to docs so people understand what values commitish accepts under target_commitish -> https://developer.github.com/v3/repos/releases/#create-a-release

[ryanweber88]

Fantastic! I'd also recommend adding this to the example further down in the README.