Document need to AcceptTermsOfService in apps

Closes #3040.

readthedocs/basic/next-steps.rst

@@ -20,3 +20,27 @@ that are worth learning and understanding.
 
 From now on, you can keep pressing the "Next" button if you want,
 or use the menu on the left, since some pages are quite lengthy.
+
+A note on developing applications
+=================================
+
+If you're using the library to make an actual application (and not just
+automate things), you should make sure to `comply with the ToS`__:
+
+    […] when logging in as an existing user, apps are supposed to call
+    [:tl:`GetTermsOfServiceUpdate`] to check for any updates to the Terms of
+    Service; this call should be repeated after ``expires`` seconds have
+    elapsed. If an update to the Terms Of Service is available, clients are
+    supposed to show a consent popup; if accepted, clients should call
+    [:tl:`AcceptTermsOfService`], providing the ``termsOfService id`` JSON
+    object; in case of denial, clients are to delete the account using
+    [:tl:`DeleteAccount`], providing Decline ToS update as deletion reason.
+
+.. __: https://core.telegram.org/api/config#terms-of-service
+
+However, if you use the library to automate or enhance your Telegram
+experience, it's very likely that you are using other applications doing this
+check for you (so you wouldn't run the risk of violating the ToS).
+
+The library itself will not automatically perform this check or accept the ToS
+because it should require user action (the only exception is during sign-up).