MXS-1752: Warn that module is not GA

The module is not GA quality software so a warning should be given in the
documentation.

Documentation/Routers/Cat.md

@@ -1,6 +1,8 @@
 # Cat
 
-The `cat` router is a special router that concatenates result sets.
+The `cat` router is a special router that concatenates result sets. The module
+is still in beta testing phase so usage in production environments is
+not encouraged.
 
 ## Configuration