Install steps - Add numpy requirement in manual installation steps

When `pip install -r requirements/edx/pre.txt` is omitted before running
`pip install -r requirements/edx/base.txt`, it would cause API versions
mistmatch when `make cms` is run. `make cms` would automatically run pip
with pre.txt and install a different version of numpy, *after* base.txt
and post.txt have been run through pip.

README.md

@@ -76,6 +76,7 @@ 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 requirements/edx/pre.txt
     $ pip install -r requirements/edx/base.txt
     $ pip install -r requirements/edx/post.txt
     $ bundle install