Update extensions.md

docs/features/extensions.md

@@ -38,6 +38,8 @@ Shuttle will not reserve keywords for its own commands but, will always execute
 its own first, this effectively means that we will not respect your list of
 extensions.
 
+Notice: Extensions will be turned off if shuttle is run within plan actions (shuttle run build etc.). This is because extensions are not made to be distributed to those parts of your CI and in this case it is your plans job to make sure you have good developer experience. An extension can still use a plan, but not the other way around
+
 ## Usage
 
 To enable shuttle extensions one or more registries are required. A registry is