Note: tools that mutate the schema after construction may break the schema

Author: benjie

src/pages/postgraphile/make-process-schema-plugin.md

@@ -23,6 +23,11 @@ called with the generated schema and must either return the same schema (e.g. if
 you're doing a read-only operation, or mutating the schema directly) or return
 an alternative schema (e.g. a derivative).
 
+**PLEASE NOTE**: some third party tooling mutates the existing GraphQL schema
+which is likely to cause issues. Please use only tools that treat GraphQL
+schemas as immutable; if you cannot then try building a sacrificial schema that
+delegates to the PostGraphile schema but can be mutated.
+
 ### Example
 
 ```js