Rename Readme.md to README.md so that it shows up on npmjs.org

No documentation for this project is showing up on it's official
npmjs.org listing:

    https://npmjs.org/package/retry

Renaming Readme.md to README.md should fix that.
Document the units of minTimeout and maxTimeout to be milliseconds.

From other clues in the documentation, this appears to be the right unit.
Merge pull request #11 from markstos/master

Rename Readme.md to README.md so that it shows up on npmjs.org