Update docs a little bit

README.md

@@ -3,13 +3,32 @@
 This is a proof-of-concept start at a python version of [Ruby's VCR
 library](https://github.com/myronmarston/vcr).
 
-#What it is supposed to do
+##What it is supposed to do
 Simplify testing by recording all HTTP interactions and saving them to
 "cassette" files, which are just yaml files.  Then when you run your tests
 again, they all just hit the text files instead of the internet.  This speeds up
 your tests and lets you work offline.
 
-#Similar libraries in Python
+##Compatibility Notes
+Currently I've only tested this (a little bit) with urllib2.  It's known to *NOT
+WORK* with urllib.
+
+##How to use it
+```python
+import vcr
+import urllib2
+
+with vcr.use_cassette('fixtures/vcr_cassettes/synopsis.yaml'):
+    response = urllib2.urlopen('http://www.iana.org/domains/example/').read()
+    assert 'Example Domains' in response
+```
+
+##Ruby VCR compatibility
+I'm not trying to match the format of the YAML files.  Cassettes generated by
+Ruby's VCR are not compatible with VCR.py.  The API is similar but I don't have
+nearly as many features
+
+##Similar libraries in Python
 Neither of these really implement the API I want, but I have cribbed some code
 from them.
  * https://github.com/bbangert/Dalton