README for Gin 1.5. basic distribution

Due to a backwards incompatible change in GWT 2.2.0, Gin's 1.5 release contains
two different compiled versions: 1.5-pre-gwt-2.2 and 1.5-post-gwt2.2. These two
jars are feature-equivalent, the only difference is that each works with a
different subset of GWT versions:

* 1.5-pre-gwt-2.2 works with GWT 2.1.1 and before
* 1.5-post-gwt2.2 works with GWT 2.2.0 and later
