docs: note on Open Collective payment timing and automatic credit

getting-started.md

@@ -26,6 +26,13 @@ account. Click it and create your login.
 > as a member. (Contributed without an Open Collective account? You're recognized
 > by the email you entered at checkout.)
 
+**On payment timing:** Open Collective charges your contribution on the
+collective's billing day, not the day you joined — so your second charge can
+arrive sooner than a month after the first. When that leaves a paid month
+shorter than a month, we credit the difference automatically as non-expiring
+credits, pro-rated to the days lost. It's automatic: no need to ask, claim it,
+or contact anyone.
+
 ### Your membership includes
 
 - **Private AI chat** through the co-op's shared gateway.