Add instructions on using libyaml

For a 10x speed increase!

README.rst

@@ -573,6 +573,24 @@ If you set the loglevel to DEBUG, you will also get information about
 which matchers didn't match. This can help you with debugging custom
 matchers.
 
+Speed
+-----
+VCR.py runs about 10x faster when pyyaml can use the libyaml extensions. However, just installing ``libyaml`` (Mac) or `libyaml-dev`` (Linux) is not enough, as pyyaml needs to be rebuild with the proper flag. Note that this flag is cached by pip, so clear the cache first. 
+
+Are you using libyaml already? This should work:
+
+::
+
+    python -c 'from yaml import CLoader'
+
+If not:
+
+::
+
+    pip uninstall pyyaml
+    pip --no-cache-dir install pyyaml
+
+
 Upgrade
 -------