Document PermGen requirements for JDK7 and the embedded proxy

[rtyler]

Based on some internal testing with a JDK7 build, @mkristian and I have identified challenges with using a multi-project build with the new rubygems() repository handler.

There should be a caveat added to the docs when talking about this incubating feature on JDK7 since it hits the limited default PermGen space pretty hard.

[rtyler]

This is briefly mentioned in the 1.1 release announcement