Update Makefile.am

Fix build failure due to missing README file.

The build process fails because Makefile.am includes README as part of the documentation files, but actually it doesn't exist:

make[2]: *** No rule to make target `README', needed by `all-am'. Stop.

The right file is README.md, so making that replacement to Makefile.am fixes the problem.
Merge pull request #6 from vriera/master

Update Makefile.am