Add note about fresh subprocess per build (#308)

pep-0517.txt

@@ -318,6 +318,9 @@ If not defined, the default implementation is equivalent to ``return []``.
    pick how to make an editable install? And if the frontend does, what does it
    need from the backend to do so.
 
+Config settings
+===============
+
 ::
 
   config_settings
@@ -425,6 +428,10 @@ meets the above criteria. In particular, it MUST NOT assume that it
 has access to any packages except those that are present in the
 stdlib, or that are explicitly declared as build-requirements.
 
+Frontends should call each hook in a fresh subprocess, so that backends are
+free to change process global state (such as environment variables or the
+working directory). A Python library will be provided which frontends can use
+to easily call hooks this way.
 
 Recommendations for build frontends (non-normative)
 ---------------------------------------------------