Readme with zing

[README.md] (https://github.com/gnab/editableCell/blob/readme-with-zing/README.md)

* Add a lot of information to the README, including Getting Started, additional sections on `selection`, comparison with other libraries, and license.

[Demo](https://github.com/gnab/editableCell/blob/readme-with-zing/index.html)

* Update demo page with additional help text and use of cellHTML binding. Link above, however, is to the commit, not a runnable version.

Would appreciate a read-through of the new README from @gnab and @espenh to make sure I didn't make any dumb mistakes!