Add context to README about transition from Mozilla to jsfiddle

Last time I looked, togetherjs lived with us in the https://github.com/mozilla organization . It's since been transferred to the https://github.com/jsfiddle organization. Looking in issues and code I don't see any information as to when or why this was done.

Do add some background on the project (about how it came out of Mozilla Labs, was put on a shelf in 2014 and then at some point in the last couple years was transferred to jsfiddle)