Extend the README file

[danielmewes]

The readme should mention
a) what the demo is about (it just says "Examples using Node.js and RethinkDB" currently)
b) how to run the example
c) how to configure which server to connect to (edit the config.js file and point it to a running RethinkDB server)

(related to #4)

[danielmewes]

Actually I only saw the global README. There are much better README.ms files in the individual implementation directories.

We should still improve the global README to describe what's in this repository and make it link to the other READMEs.

We should also clarify the other READMEs to say how to configure the application and that a RethinkDB server must be started first.
Here for example https://github.com/rethinkdb/rethinkdb-example-nodejs/tree/master/todo-angular-express we just say "Start the server with node app.js" which I can see to be confusing if you don't know that you also need to start the RethinkDB server separately.