Rename README to README.md and fix it to make it  pretty-printed

Of course, I know README was copied from https://code.google.com/p/scala-migrations/ .  I send this pull request to make the copied document pretty-printed on github.

* [Github Flavored Markdown](https://help.github.com/articles/github-flavored-markdown) is used to do it
* The rendered result of the document can be seen from https://github.com/kmizu/scala-migrations/tree/fix-readme