Slack button accesses remote js file, which would be better run locally

[bhny]

It would be good to run the slackin.js file locally if possible.
(currently https://slack.nycmesh.net/slackin.js)

If not possible, can we document in the repo where file is, who's server and how to edit etc.

[jedahan]

As a temporary (read: probably tempermanent) measure, I added a little documentation about the slack button to the readme in #27

[jedahan]

I'm a little oversubscribed to stuff, anyone else can feel free to pick this one up