Update CONTRIBUTING.md

CONTRIBUTING.md

@@ -24,14 +24,13 @@ version of objects in other cases.
 
 When adding a new function or modifying an exiting one make sure that the
 [VOLATILITY](https://www.postgresql.org/docs/current/static/xfunc-volatility.html) and [PARALLEL](https://www.postgresql.org/docs/9.6/static/parallel-safety.html) categories are updated accordingly.
-As PARALLEL labels need to be stripped for incompatible PostgreSQL versions
-please use _PARALLEL SAFE/RESTRICTED/UNSAFE_ in uppercase so it's handled
-automatically.
-
-When used as an extension (probably always from version 0.2.0 onwards)
-all the objects will be installed in a "cartodb" schema. Take this into
-account to fully-qualify internal calls to avoid (possibly dangerous)
-name clashes.
+
+
+Although the extension will usually be installed in the "cartodb" schema, please
+use @extschema@  to fully-qualify internal calls to avoid name clashes.
+When you use postgis functions or types, please fully-qualify them by using
+@postgisschema@ (it's changed to "public" by the install script) to avoid
+pg_upgrade issues.
 
 Every new feature (as well as bugfixes) should come with a test case,
 see the 'Writing testcases' section.