readme: add example usage

Author: emersion

README.md

@@ -6,6 +6,15 @@ Python library to parse and format email threads. Give it a list of emails that
 are part of the same thread and it'll build a tree of responses to the original
 message.
 
+```python
+import emailthreads
+import mailbox
+
+mbox = mailbox.mbox("/path/to/mbox/thread")
+thread = emailthreads.parse(mbox)
+print(thread)
+```
+
 ## Tests
 
 To run the tests, execute this command: