[analyzer] Add a doc describing the internals of RegionStore.

This is a text file with Markdown-ish formatting because we haven't decided
where analyzer internal documents should go, but it's probably better to
have this in source control than sitting on my local drive forever.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@174398 91177308-0d34-0410-b5e6-96231b3b80d8