Add Slackin badge

[wuworkshop]

Thought this Slackin invite widget was nice on Scotch.io.

http://rauchg.com/slackin/

[abdulhannanali]

@wuworkshop We already have it set up at http://codebuddiesmeet.herokuapp.com/
@lpatmo please!

[wuworkshop]

@abdulhannanali did you look at that Slackin link? We don't actually have that widget on codebuddies.org. I'm talking about the Slackin widget that displays how many users are online out of total users. It looks like this: