README: delete section on jdkhome files.

These have not been used since commit
cf5e1fb73cca809bcd00fcce6ecb3c5e9dce5594.
README: add note about ANT_HOME.

This section is based on experience building on ArchLinux
where ANT_HOME appeared to be being set to indicate the
system installation of ANT.  This prevents it from finding
the local modifications.
README: update links to repository.