Update README.md with AMD specific instructions

Adding details about Sinon.JS not working when loaded asynchronously by AMD loaders. Hopefully this will cut down on the amount of issues created by people trying to use it that way ... or at least provide a linkable explanation that is easy to paste when closing these issues.
Merge pull request #553 from mroderick/update-readme-about-amd-and-source

Update README.md with AMD specific instructions