Add a PORTING document which describes some major API differences.

There are a few things that differ in ways that are not source-compatible, so
it's probably worth documenting them.

Change-Id: I4ef26173a9347d9fd517c1b5215e08ced660b79d
Reviewed-on: https://boringssl-review.googlesource.com/5788
Reviewed-by: Adam Langley <agl@google.com>