Add CMAKE doc to describe SHARED lib policy

[philburk]

Describe how to build a shared library that links PortAudio statically.
-BUILD_SHARED_LIBS=ON -BUILD_SHARED_LIBS=OFF

See #1085.
Do this after #1044 merged.

[RossBencina]

I think the main question, aside from being consistent with the actual cmake behavior is whether we need to describe that we respect BUILD_SHARED_LIBS if PA_BUILD_SHARED_LIBS is not set. And maybe the use-cases where you use PortAudio as as a sub-project where you want to control shared on/off independently from global BUILD_SHARED_LIBS behavior. But I think that is mostly already handled by the new docs at #1044