Add a note about the docs versioning

Author: bbatsov

doc/antora.yml

@@ -1,5 +1,7 @@
 name: cider-nrepl
 title: CIDER nREPL
+# The version should be in the format major.minor (e.g. 0.28).
+# Don't include patch in the version (e.g. 0.28.1).
 version: ~
 nav:
 - modules/ROOT/nav.adoc