Improve readme

Author: kapetan

README.md

@@ -1,3 +1,20 @@
 # ssh-private-key-buildpack
 
-Configure ssh to use provided key in `SSH_KEY` environment variable.
+A heroku buildpack for setting the ssh private key as part of the application build. It's meant to be used with [heroku-buildpack-multi](https://github.com/heroku/heroku-buildpack-multi), before other buildpacks which require the key to be present, like installing private `npm` modules from `github`.
+
+# Example usage
+
+Upload the private key to heroku (note that the key needs to be base64 encoded).
+
+```
+heroku config:set SSH_KEY=$(cat ~/.ssh/id_rsa | base64)
+```
+
+Add a `.buildpacks` file (used by `heroku-buildpack-multi`) which contains this and the default node.js buildpack.
+
+```
+https://github.com/debitoor/ssh-private-key-buildpack.git#v1.0.0
+https://github.com/heroku/heroku-buildpack-nodejs.git#v75
+```
+
+Now as long as the public key is present on github and the user has the correct permissions, it's possible to install `npm` modules from private `githup` repositories.