Add a note about user-env-compile

README.md

@@ -18,6 +18,19 @@ buckets. It gives you a way of deploying pre-built code to
 You probably want to use an [IAM key](http://aws.amazon.com/iam/) with limited
 access. This code only requires `s3:GetObject` access to files.
 
+If you don't want to check your IAM keys into revision control, you can use
+the [Heroku Labs user-env-compile feature](https://devcenter.heroku.com/articles/labs-user-env-compile)
+to store the keys in Heroku's config system. Note that this is a beta feature
+and Heroku may remove it at any time:
+
+    $ heroku config:add BUILDPACK_URL=https://github.com/paulhammond/s3-tarball-buildpack.git
+    $ heroku labs:enable user-env-compile
+    $ heroku config:add AWS_ACCESS_KEY_ID=AKIA0000000000000000
+    $ heroku config:add AWS_SECRET_ACCESS_KEY=xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
+
+    $ cat .buildpack-s3-tarballs
+    s3://bucket/path/to/tarball.tgz
+
 In most cases you'll use this buildpack in conjunction with other buildpacks
 using [heroku-buildpack-multi](https://github.com/ddollar/heroku-buildpack-multi):