Fixes automake error caused by the commit 9635494bf.

The commit 9635494bf renamed README with README.md, leading to
the situation automake is complaining that there is no `README`
in this project unless we specify the option `foreign` into Makefile.am.
Merge pull request #5 from tetsuok/fix-autotools-readme-not-found-error

Fixes automake error caused by the commit 9635494bf.