Add document of changes between Android versions.

This document does not list every change in the unwinder between
Android versions. Instead it attempts to only include information
that someone building an app targeting a specific version of Android
might need to know to make sure their app can unwind properly.
It also tries to describe the way that stacktraces might be displayed
differently between Android versions.

Test: NA
Change-Id: I4029053f763f3471f7ddb5da9b1de2d325ead455