docs: Document that ehrQL follows cohort-extractor

It is not immediately obvious to someone encountering this repository
outside of the context of OpenSAFELY (for example, reading the
documentation) that active development is on ehrQL, which replaces
cohort-extractor for new OpenSAFELY studies.

README.md

@@ -1,3 +1,12 @@
+# cohort-extractor is in maintenance mode
+
+Active development is focused on
+[ehrQL](https://github.com/opensafely-core/ehrql),
+the electronic health record query language
+that is cohort-extractor's successor.
+
+Refer to the [ehrQL documentation](https://docs.opensafely.org/ehrql/) to learn more.
+
 # OpenSAFELY cohort extractor tool
 
 This tool supports the authoring of OpenSAFELY-compliant research, by: