First pass at a CONTRIBUTING guidelines document.

* Added CONTRIBUTING.md document
* Updated README.md
  * Include link to Contributing guidelines
  * Add some more info to make a little less minimal
  * Add some links where appropriate
  * Extend paragraph to make it clear the local site booting in dev mode
    won't be able to talk to Cosm.
Merge pull request #39 from smulube/contributing_guidelines

First pass at a CONTRIBUTING guidelines document.
Find datastreams by tags not id - resolves #41

Datastream ids are now variable and include each device MAC code. This
means we cannot find them simply based on the id, instead we need to
detect the correct datastream by inspecting the tag contents to find the
appropriate match.

This commit also adds a spec helper which returns some 'real' JSON as
currently being published to Cosm be an Egg, and updates the dashboard
spec so that we test for the presence of the detected values.
Merge pull request #42 from smulube/datastream_finders

Find datastreams by tags not id - resolves #41