[IMPORTANT] Fieldable and LUCENE-1349

[Grant Ingersoll]

Per https://issues.apache.org/jira/browse/LUCENE-1349, we have made an  
exception to Lucene's backward compatibility rules and marked  
Fieldable as "changeable", namely meaning we will allow, on a case-by- 
case basis, changes to the interface, meaning anyone who implements  
there own Fieldable (which we suspect is very, very few people) may  
have to make code changes when upgrading within a minor version.  More  
than likely, Fieldable will be deprecated and changed for 3.0 (when we  
get there.)

This is noted prominently in CHANGES.txt and on the interface.  Sorry  
for the inconvenience.

Thanks,
Grant