Update readme

[mayeaux]

Explain bower is a dep for npm install
Explain using electron-prebuilt

Add new developers
Change the link directly to the heroku app for getting a slack invite

[SaturnusDJ]

Yeah if bower is not automatically installed by npm install it should be explained. Then again, @hanford was working on removing bower so might not be needed to explain bower.

[mayeaux]

Takes only 30 seconds to add it to the readme. I'll get this done sometime soon

[amosshapira]

Yep, I'm not familiar with the npm eco-system and had to figure out to manually execute npm install bower from the error message I got from npm install while I was following the instructions in the README.

I don't see any downside for adding this step until Bower dependency is removed.

[gdborton]

I believe that this can be closed. I see that bower is in the README.