Include README.rst instead of README in manifest (eliben#247)

`setup.py bdist_wheel` warns about not finding README.

Author: olofj

MANIFEST.in

@@ -2,7 +2,7 @@ recursive-include elftools *.py
 recursive-include scripts *.py
 recursive-include examples *.py *.elf *.out
 recursive-include test *.py *.elf *.arm *.mips *.o
-include README
+include README.rst
 include LICENSE
 include CHANGES
 include tox.ini