use binder folder for repo2docker config

[devvyn]

As per: http://repo2docker.readthedocs.io/en/latest/usage.html#preparing-your-repository

At the same time, it makes sense to split up the README so the appropriate focus is possible in context.

This would get us:

/
|
+--binder/
|   |
|   +--README
|   |
|   +--requirements.txt
|   |
|   +--postBuild
|
+--notebook/
|   |
|   +--README
|   |
|   +--projects/
|
+--README