Short README on gdb support.

Misc/README.gdb

+gdb Support for Python
+======================
+
+gdb 7 and later
+---------------
+In gdb 7, support for extending gdb with Python was added. When CPython is
+built you will notice a python-gdb.py file in the root directory of your
+checkout. Read the module docstring for details on how to use the file to
+enhance gdb for easier debugging of a CPython process.
+
+
+gdb 6 and earlier
+-----------------
+The file at ``Misc/gdbinit`` contains a gdb configuration file which provides
+extra commands when working with a CPython process. To use the file, either
+copy the commands to your personal gdb configuration file or symlink
+``~/.gdbinit`` to ``Misc/gdbinit``.