clarify in README that replication is required for autoincrementing IDs

README.md

@@ -20,6 +20,20 @@ package tutorial
 import "rockfic.com/orm"
 ```
 
+## Convert your database to a replica set (if you haven't already)
+
+this is **very important!!** otherwise, functionality like
+auto-incrementing IDs will not be available.
+
+add these lines to `mongod.conf`:
+
+```
+replication:
+    replSetName: "rs0"
+```
+
+of course, you can replace `rs0` with whatever name you want.
+
 ## Connect to the database
 
 ```go