a couple readme additions

It looks like the plugin has been added to JCenter.  I've updated the readme to include instructions on how to import the plugin directly using JCenter.  Internally, we use an Artifactory instance that proxies JCenter, but I expect that the organizations that have something like that set up will already be aware of how to configure it.

The one additional gotcha I've encountered with the plugin is with regards to handling signed JARs.  I documented the workaround that we're using in the readme as well.