Draft NOTES for 1.13.2
Add an include in serialize_datum.cc.

This fixes an error I encountered when building on Mac OS X with clang in release mode. The error was that 'signbit()' was not defined. I don't know what other build scenarios this error might occur in.

This change has been reviewed by Daniel Mewes.
Merge v1.13.3 into v1.13.x