Document changes to OS X installer configurations for 2.7.7.

Document changes to OS X installer configurations for 2.7.7.
As of 2.7.8, the 32-bit-only installer will support OS X 10.5
and later systems as is currently done for Python 3.x installers.

For 2.7.7 only, we will provide three installers:
the legacy deprecated 10.3+ 32-bit-only format;
the newer 10.5+ 32-bit-only format;
and the unchanged 10.6+ 64-/32-bit format.

Although binary installers will no longer be available from
python.org as of 2.7.8, it will still be possible to build from
source on 10.3.9 and 10.4 systems if necessary.