Move README.gdb to the devguide.

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``.