Adds dev guide to README

Felt like undergrads in the lower years would struggle finding their way
around the files if they've had no experience with ruby, rails,
coffeescript or sort of server. Not patronising, just know what I
would've liked to know in order to get stuck in quicker.

Also edited todo list & appended link to timeline (hope you don't mind,
it's not even that great. Just hoping I'll eventually make it better
haha)