[DOC] appendix with guidance on updating applications for HBase 2

Feedback from mdrob on HBASE-19158:

Classes/Methods moved:

• HBASE-19425 (and subtasks) Put/Delete/Increment/Append
• HBASE-19627 (and subtasks) Cell & Impl
• HBASE-18978 (and subtasks) Async/Table
• HBASE-18805 (and subtasks) Async/Admin
• HBASE-14996 (and subtasks) Lots of misc changes
• HBASE-13197 (and subtasks) Connection
• HBASE-13844 KeyValue -> CellUtil
• Maybe HBASE-19950 is worth calling out? Not really an upgrade issue though, but I know the old behavior of SCVF changed in a surprising way for the YARN ATS folks, when they were previously relying on what ended up being a bug.

Could be mitigated by having a compatibility report, but would benefit from narrative messaging.

Almost definitely need a blanket reminder that some deprecated APIs went away.

from @busbey on HBASE-19158

* Classes/Methods moved:

* Almost definitely need a blanket reminder that some deprecated APIs went away.

My intention was for both of these to be handled by this blurb:

+[[upgrade2.0.public.api]]
+.Multiple breaking changes to source and binary compatibility for client API
+The Java client API for HBase has a number of changes that break both source and binary compatibility for details see the Compatibility Check Report for the release you'll be upgrading to.
+
+<!-- This would be a good place to link to an appendix on migrating applications -->
+

I figure once we get an appendix that provides guidance on migrating an application from 1.x to 2.x we can also link to it.

Get an appendix in place and then update the upgrade section as needed to draw attention to it.