Make it slightly clearer how to enable authentication.

The README previously sort of covered this, but in particular it was not
obvious to me that the request parameter that corresponded to enabling
`apiKey` is called `key`, and not `apiKey`, which ultimately made me
have to look at the source code to figure this out (even though the word
`key` is mentioned elsewhere on the page, it still wasn't very easy to
make the connection).

This section puts all the required information in one spot.

Author: Julian

README.md

@@ -156,6 +156,13 @@ const result = await invoke('deckNames', 6);
 console.log(`got list of decks: ${result}`);
 ```
 
+### Authentication
+
+Anki-Connect supports requiring authentication in order to make API requests.
+This support is *disabled* by default, but can be enabled by setting the `apiKey` field of Anki-Config's settings (Tools->Add-ons->AnkiConnect->Config) to a desired string.
+If you have done so, you should see the [`requestPermission`](#requestpermission) API request return `true` for `requireApiKey`.
+You then must include an additional parameter called `key` in any further API request bodies, whose value must match the configured API key.
+
 ### Hey, could you add a new action to support $FEATURE?
 
 The primary goal for Anki-Connect was to support real-time flash card creation from the