Fix broken links in README.rdoc

[pganguli]

Notes:

• The bitly link just 404's. Not sure what it originally pointed to.
• {Dropbox}[...]-like results in a malformed link. Apparently the space after ']' is necessary.
• You should probably check for broken links to other pages in your blog as well.
• GitHub renders the image in-place, and uses the link text as the alt-text, so changed that to 'diagram'.