blurb about transfer categories (#283)

We get lots of questions about transfers involving off budget accounts.
This will hopefully alleviate some of that confusion.

It was mentioned in
[actualbudget/#1979](actualbudget/actual#1979)
that the docs don't have a good description of this.

docs/transactions/transfers.md

@@ -18,6 +18,12 @@ If you are importing files, it is recommended to import into a single account an
 
 Delete a transfer the same way you delete any transaction: hover over it and press the **X** on the left. Deleting a transfer transactions always deletes both transactions in each account.
 
+## Transfer Categories
+
+Transfers between On Budget accounts don't have a category.  This is because the funds never left your budget.  One way to think about this is that the funds both left and entered your budget thus canceling each other out. Actual marks these types of transfers with _`Transfer`_ in the category field.
+
+Transfers between Off Budget and On Budget accounts are different.  Only one half of the transfer exists on your budget and, from the perspective of your budget, is the same as a regular transaction.  You can create these transfers just the same as an On Budget transfer, but you will be asked to assign a category on the On Budget side of the transfer.
+
 ## Payee rules
 
 Transfers are actually just custom payees, so if you want to you can [create custom rules for them](./payees.md#transfer-payees).