Instructions for developers in README.md are incorrect

Created by: davemevans

The instructions for developers (https://github.com/Dash-Industry-Forum/dash.js/tree/development#reference-player) are a bit misleading as they imply that you can try out the reference player on the development branch without building the code, which isn't true.

The README.md should be updated to say that either you need to checkout the master branch (which includes the dist folder), or make it clear that you have to build the library first.