adding reference to subquery argument requirement

.github/PULL_REQUEST_TEMPLATE.md

@@ -3,4 +3,5 @@
 - [ ] Include python is activated for new functions. Include this before importing modules: `plpy.execute('SELECT cdb_crankshaft._cdb_crankshaft_activate_py()')`
 - [ ] Docs for public-facing functions are written
 - [ ] New functions follow the naming conventions: `CDB_NameOfFunction`. Where internal functions begin with an underscore `_`.
+- [ ] If appropriate, new functions accepts an arbitrary query as an input (see [Crankshaft Issue #6](https://github.com/CartoDB/crankshaft/issues/6) for more information)