setup.py: install *.rst files instead of *.txt, trying to fix #279

It seems that since commit 8e0d3bb5 txt documents are rewrited using
restructured text, but setup.py wasn't updated to install them.

This commit updates setup.py to install files with rst extension
instead.

Signed-off-by: Ｖ字龍(Vdragon) <pika1021@gmail.com>
Merge pull request #280 from Vdragon/fix_missing_rst

setup.py: install *.rst files instead of *.txt

Closes #279

Signed-off-by: David Aguilar <davvid@gmail.com>