add README.md to fragment-layout directory.

Document, right next to where one makes these edits, 
* usage of `username` attribute rather than implying username from file name.
* Node ID uniqueness constraint beyond that expected of `ID` attribute in XML, with example ala [Tim Raymond's suggestion on `uportal-user@`](http://thread.gmane.org/gmane.comp.java.jasig.uportal/16949/focus=16958).
* Thoughtfulness advisable in re-using and avoiding re-use of node `ID`s.