MAINT: deprecate MySQL support.

CHANGELOG.md

@@ -13,6 +13,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Deprecated ###
 
+* MySQL Support. Last version which is known to work well with LinkAhead is MySQL 5.6
+
 ### Removed ###
 
 ### Fixed ###

README_SETUP.md

 # Setup of the CaosDB SQL back end
 
 ## Dependencies
-* `MariaDB Client 10.1`, `MySQL Client 5.5`, or later versions.  In the case of
-  MySQL, version 5.6 is recommended.
-- make
+
+* `MariaDB Client 10.1` or later, `MySQL Client >=5.5, <=5.6`.
+* make
 
 ## Create the configuration
 * Create an empty `.config` file. For the default values and the meaning of