Update README.md to add description of sbt-apigen project.
Update README.md to add description of sbt-apigen project.

Seanstoppable/apidoc,Seanstoppable/apidoc,apicollective/apibuilder,gheine/apidoc,movio/apidoc,mbryzek/apidoc,Seanstoppable/apidoc,apicollective/apibuilder,movio/apidoc,movio/apidoc,apicollective/apibuilder,mbryzek/apidoc,mbryzek/apidoc,gheine/apidoc,gheine/apidoc