docs: add link to configuration spec

[steveoh]

I had no idea what to put in the .versionrc until i stumbled upon the spec. A sample for how to do it in the package.json would be a great resource also but I don't know how to do that from reading the docs.

It is still unclear to me if it is only cli options are available or if the options in link I provided also work.

[coveralls]

Coverage remained the same at 100.0% when pulling 8672c01 on steveoh:patch-1 into c9cb339 on conventional-changelog:master.

[bcoe]

@steveoh any options available in the configuration spec should also be available as command line options 👍

Thanks for adding this documentation.

Please let me know if there's anything else we can add to the docs to help clarify, and I'll happily land more patches; appreciate the contribution.