Update README.txt in _mcl_autogroup

mods/CORE/_mcl_autogroup/README.txt

@@ -4,6 +4,11 @@ Specifically, this mod has 2 purposes:
 1) Automatically adding the group “solid” for blocks considered “solid” in Minecraft.
 2) Generating digging time group for all nodes based on node metadata (it's complicated)
 
+This mod also requires another mod called “mcl_autogroup” to function properly.
+“mcl_autogroup” exposes the API used to register digging groups, while this mod
+uses those digging groups to set the digging time groups for all the nodes and
+tools.
+
 See init.lua for more infos.
 
 The leading underscore in the name “_mcl_autogroup” was added to force Minetest to load this mod as late as possible.