README: Document template variables.

README.rst

@@ -65,6 +65,35 @@ Patreon
 ``template patreon svcfees``
   Imports one expense transaction per month for that month's Patreon service fees.  Generated from Patreon's earnings report CSV.
 
+Template variables
+~~~~~~~~~~~~~~~~~~
+
+import2ledger templates have access to a few variables for each transaction that can be included in the output entry using `Python's format string syntax <https://docs.python.org/3/library/string.html#formatstrings>`_.  For most uses, all you need to do is write ``{variable_name}`` in the template, and the variable's value will be filled in there.  For example, this template sets different payees for each side of the transaction::
+
+  template patreon income =
+    Income:Patreon  -100%%
+    ;Payee: {payee}
+    Accrued:Accounts Receivable:Patreon  100%%
+    ;Payee: Patreon
+
+You can use the following variables:
+
+================== ==========================================================
+Name               Contents
+================== ==========================================================
+amount             The total amount of the transaction, as a simple decimal
+                   number (not currency-formatted)
+------------------ ----------------------------------------------------------
+currency           The three-letter code for the transaction currency
+------------------ ----------------------------------------------------------
+date               The date of the transaction, in your configured output
+                   format
+------------------ ----------------------------------------------------------
+payee              The name of the transaction payee
+================== ==========================================================
+
+Custom hooks may add more template variables.
+
 Other output options
 ~~~~~~~~~~~~~~~~~~~~