Recently bpf/ docs were converted to us RST format. 'README.rst' was
created but in order to fit in with the Sphinx build system this file[...]
should be named 'index.rst'. Rename file, fixes to integrate into Sphinx
build system in following patches.
docs: Rename Documentation/bpf/README.rst to Documentation/bpf/index.rst
Signed-off-by: Tobin C. Harding <me@xxxxxxxx>