Update index.md

Signed-off-by: James Cole <james@firefly-iii.org>

Author: JC5

docs/docs/references/firefly-iii/api/index.md

@@ -10,6 +10,10 @@ Please visit the [dedicated Swagger documentation](https://api-docs.firefly-iii.
 
 The API is slated to become the only way to talk to Firefly III: even the UI would use the API exclusively. Alas, this is a work in progress.
 
+## Developing with the Firefly III API
+
+If you want to keep up with changes to the Firefly III API, be sure to sign up to [the Firefly III dev mailing list](https://firefly-iii.kit.com/dev). It's free and low-frequency.
+
 ## Currencies in the API
 
 All objects where currency is relevant have `currency_*` attributes. They refer to the currency set in the object. Most objects (accounts, transactions, etc.) carry their own currency settings. For example, a bill or a piggy bank has its own currency, regardless of the primary currency of the administration. Expect something like this:
@@ -52,4 +56,4 @@ When you create a webhook you can choose webhook-specific data to be the respons
 
 However, when you create a webhook that ALSO triggers on the creation of a new budget, you cannot set the transaction details as the expected response. In that case, you must select the option "The relevant details" (`RELEVANT`) which will automatically select the correct data to send as response.
 
-The reason this is complicated is because some webhook triggers come with multiple possible responses, so it is always up to you to select what you need. However, for some trigger combinations `RELEVANT` is the only possible response. 
+The reason this is complicated is because some webhook triggers come with multiple possible responses, so it is always up to you to select what you need. However, for some trigger combinations `RELEVANT` is the only possible response.