diff --git a/Readme.md b/Readme.md
index 87ee263..2fe6008 100644
--- a/Readme.md
+++ b/Readme.md
@@ -116,3 +116,12 @@ This allows to:
 - see the counter for the websocket connections
 - set a status message which is shown at all linked clients at the overview screen
 - unlock the premium version feature for a user for a month or a year
+
+## Purchases
+
+To enable the automated purchase feature, set the ``GOOGLE_PLAY_PUBLIC_KEY`` environment variable.
+The value for the official builds which are distributed using the Play Store can
+be found at <https://codeberg.org/timelimit/timelimit-android/src/commit/3da677877f4dde0b1b01523daae33745f14e08ac/app/build.gradle#L49>.
+
+Additionally, there is the admin API which allows one to unlock the
+premium features.