adds readme detail

README.md

@@ -18,9 +18,11 @@ const myOtherDb = new PouchDB(
   'http://mysite:5984/mydb', 
   { auth: { username: 'admin', password: 'pass' } 
 });
+const sessionDb = new PouchDB('http://mysite:5984/mydb', { session: 'existent session cookie' });
 
 await myDb.allDocs();
 await myOtherDb.allDocs();
+await sessionDb.allDocs();
 ```
 
 ## Overview
@@ -40,6 +42,8 @@ It supports authentication embedded in the CouchDb URL or as an additional optio
 
 It regenerates the session cookie on expiry and retries the last request, the client should not expect a failed request for an expired cookie.
 
+When given a `session` parameter, a new session will not be requested, instead the passed session cookie will be used as session authentication. 
+
 ### Testing
 
 Testing requires `docker` and `docker-compose` to launch a CouchDb 3.3.3 container.