Document DATABASES "OPTIONS"

Author: timgraham

README.md

@@ -40,10 +40,16 @@ DATABASES = {
         "NAME": "my_database",
         "USER": "my_user",
         "PASSWORD": "my_password",
+        "OPTIONS": {
+            ...
+        },
     },
 }
 ```
 
+`OPTIONS` is an optional dictionary of parameters that will be passed to
+[`MongoClient`](https://pymongo.readthedocs.io/en/stable/api/pymongo/mongo_client.html).
+
 ## Known issues and limitations
 
 - The following `QuerySet` methods aren't supported: