Merge pull request #1391 from opensafely/add-ehrql-note-to-federation…

…-page

Add note about ehrQL to federation page

docs/federation.md

@@ -32,7 +32,12 @@ We have various [examples](#examples) of completed and published analyses that w
 
 The researcher wrote the code as a **single codebase**. As with all OpenSAFELY activity, this is composed of small, cross-backend components called *actions*.
 
-The first action they wrote extracts a dataset using an [OpenSAFELY study definition](study-def.md)—a uniform way of extracting variables from very different databases.  For example, this section of the study definition creates a variable that indicates when a patient was most recently recorded as having severe obesity:
+The first action they wrote extracts a dataset using an [OpenSAFELY study definition](study-def.md)—a uniform way of extracting variables from very different databases.
+(Note that this example uses cohort-extractor, which has been deprecated in favour of [ehrQL](/ehrql/).
+However, this doesn't change the approach to federation.)
+
+For example, this section of the study definition creates a variable that indicates when a patient was most recently recorded as having severe obesity:
+
 
 ```python