updating readme

Readme.md

@@ -3,6 +3,8 @@ Python Build Pack
 The Python Build Pack (PBP) is a build pack for running Python and Django
 apps on Heroku.
 
+## Usage
+
 If `requirements.txt` is present, the PBP considers the directory a Python app
 with packages to install via pip.
 
@@ -11,11 +13,13 @@ directory a Python/Django app, and patches `settings.py` to parse the Heroku
 DATABASE_URL config vars. It then sets default process types to use the Django
 web server.
 
-External Build Pack
-----------------------
-Cutting-edge development is taking place in this repo. To use this on Heroku,
-set a `BUILDPACK_URL` config var with a fully authenticated URL to this repo:
 
-    $ heroku config:add BUILDPACK_URL=https://nzoschke:XXXXXXX@github.com/heroku/heroku-buildpack-python.git
+## Hacking
+
+To change this buildpack, fork it on GitHub. Push up changes to your
+fork, then create a test app with `--buildpack YOUR_GITHUB_URL` and
+push to it. If you already have an existing app you may use
+`heroku config add BUILDPACK_URL=YOUR_GITHUB_URL` instead.
+
+For example, you could adapt it to use pypy at build time.... to be continued
 
-On next push, slug-compiler will use this BP instead of the built-in one.
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