BUILDING.md: Added info about incremental compiling, cleaning after switching branches

When I started developing with JRuby I didn't know how to do a quick incremental compilation.  I ran "mvn package" after every change to any Java code.  Thankfully, @mkristian told me how to do it in this comment:

https://github.com/jruby/jruby/issues/1470#issuecomment-33900740

@mkristian also said it would be necessary to clean the build after switching branches (i.e. from master to jruby-1_7) and I have confirmed that it is necessary with an experiment, as discussed in issue #1470.

I think it would be helpful to other new JRuby developers if this information was in BUILDING.md, so I made this pull request to add it.  I really don't know much about Maven or Java compilation but hopefully I got this right.

This pull request goes to the jruby-1_7 branch, which I think is the right way to do it.  It should be merged into master too so people can see it easily on github.