Document the hexadecimal casing thing in PORTING.md.

mab@ seems to be dealing with a fair number of these, so it's probably worth
adding to the list.

Change-Id: Ifaea3c96e7b089f28a87c7728ceb8c671786eb27
Reviewed-on: https://boringssl-review.googlesource.com/6030
Reviewed-by: Adam Langley <agl@google.com>