README: fix dependent branch names
README: fix dependent branch names

As we currently depend on the master branches of meta-oe and oe-core
adapt the README accordingly.

Furthermore add a short note on our stable branches.

Signed-off-by: Richard Leitner <0f2fdd4d2dfbb4dde6236fd550af0a2071b34781@skidata.com>
Signed-off-by: Maxin B. John <4470e5a3cfbc7ff75506df1ae89969a0d8cce1df@intel.com>

rehsack/meta-java,rehsack/meta-java,rehsack/meta-java,rehsack/meta-java,rehsack/meta-java