diff --git a/README.md b/README.md
index f1532d53b6..76dbe4b150 100644
--- a/README.md
+++ b/README.md
@@ -1,6 +1,6 @@
 This is edX, a platform for online higher education. The project is primarily
 written in [Python](http://python.org/), using the
-[Django](https://www.djangoproject.com/) framework. We also use some 
+[Django](https://www.djangoproject.com/) framework. We also use some
 [Ruby](http://www.ruby-lang.org/) and some [NodeJS](http://nodejs.org/).
 
 Installation
@@ -77,6 +77,7 @@ environment), and Node has a library installer called
 Once you've got your languages and virtual environments set up, install
 the libraries like so:
 
+    $ pip install -r pre-requirements.txt
     $ pip install -r requirements.txt
     $ bundle install
     $ npm install
diff --git a/pre-requirements.txt b/pre-requirements.txt
index 7ecead0ce7..d39199a741 100644
--- a/pre-requirements.txt
+++ b/pre-requirements.txt
@@ -1,2 +1,10 @@
+# We use `scipy` in our project, which relies on `numpy`. `pip` apparently
+# installs packages in a two-step process, where it will first try to build
+# all packages, and then try to install all packages. As a result, if we simply
+# added these packages to the top of `requirements.txt`, `pip` would try to
+# build `scipy` before `numpy` has been installed, and it would fail. By
+# separating this out into a `pre-requirements.txt` file, we can make sure
+# that `numpy` is built *and* installed before we try to build `scipy`.
+
 numpy==1.6.2
 distribute>=0.6.28