Add essentialFiles message anchor (#90)

* Add message anchor
  * For when a file a mod needs to function is overwritten with a
    potentially incompatible file.

masterlist.yaml

@@ -41,6 +41,10 @@ prelude:
       type: warn
       content: 'When using **%1%**, it''s recommended that you deactivate or delete this ESP file but keep the resources (e.g. meshes, textures) installed with this mod.'
 
+    - &essentialFiles
+      type: warn
+      content: 'Another mod seems to be overwriting one of this mod''s essential files. Please ensure you''re using this mod''s version of %1% or a compatible version if available.'
+
     - &includesX
       type: say
       content: '%1% is included with this mod.'