Add a little more info to README

README.md

@@ -5,6 +5,13 @@
 
 **jupyter-repo2docker** is a tool to build, run and push docker images from source code repositories.
 
+
+## Pre-requisites
+
+1. Docker to build & run the repositories. The [community edition](https://store.docker.com/search?type=edition&offering=community)
+   is recommended.
+2. Python 3.4+.
+
 ## Installation
 
 To install from pypi, the python packaging index:
@@ -18,5 +25,67 @@ To install from source:
 ```bash
 git clone https://github.com/jupyter/repo2docker.git
 cd repo2docker
-python3 -m pip install .
+pip install .
 ```
+
+## Usage
+
+The core feature of repo2docker is to fetch a repo (from github or locally), build a container
+image based on the specifications found in the repo & optionally launch a local Jupyter Notebook
+you can use to explore it.
+
+Example:
+
+```bash
+jupyter-repo2docker https://github.com/jakevdp/PythonDataScienceHandbook
+```
+
+After building (it might take a while!), it should output in your terminal something like:
+
+
+```
+    Copy/paste this URL into your browser when you connect for the first time,
+    to login with a token:
+        http://0.0.0.0:36511/?token=f94f8fabb92e22f5bfab116c382b4707fc2cade56ad1ace0
+```
+
+If you copy paste that URL into your browser you will see a Jupyter Notebook with the
+contents of the repository you had just built!
+
+## Repository specifications
+
+Repo2Docker looks for various files in the repository being built to figure out how to build it.
+It is philosophically similar to [Heroku Build Packs](https://devcenter.heroku.com/articles/buildpacks).
+
+It currently looks for the following files. They are all composable - you can use any number of them
+in the same repository (except for Dockerfiles, which take precedence over everything else).
+
+### `requirements.txt`
+
+This specifies a list of python packages that would be installed in a virtualenv (or conda environment).
+
+### `environment.yml`
+
+This is a conda environment specification, that lets you install packages with conda. Note that you must
+leave the name of the environment empty for this to work out of the box.
+
+### `apt.txt`
+
+A list of debian packages that should be installed. The base image used is usually the latest released
+version of Ubuntu (currently Zesty.)
+
+### `postBuild`
+
+A script that can contain arbitrary commands to be run after the whole repository has been built. If you
+want this to be a shell script, make sure the first line is `#!/bin/bash`. This file must have the
+executable bit set (`chmod +x postBuild`) to be considered.
+
+### `REQUIRE`
+
+This specifies a list of Julia packages! Currently only version 0.6 of Julia is supported, but more will
+be as they are released.
+
+### `Dockerfile`
+
+This will be treated as a regular Dockerfile and a regular Docker build will be performed. The presence
+of a Dockerfile will cause all other building behavior to not be triggered.