Add a note with adoption guidance on positional-only parameters (#38)

Author: rgommers

spec/API_specification/function_and_method_signatures.md

@@ -11,6 +11,13 @@ Function signatures in this standard adhere to the following:
    _Rationale: existing libraries have incompatible conventions, and using names
    of positional parameters is not normal/recommended practice._
 
+   .. note::
+
+       Positional-only parameters are only available in Python >= 3.8. Libraries
+       still supporting 3.7 or 3.6 may consider making the API standard-compliant
+       namespace >= 3.8. Alternatively, they can add guidance to their users in the
+       documentation to use the functions as if they were positional-only. 
+
 2. Optional parameters must be
    [keyword-only](https://www.python.org/dev/peps/pep-3102/) arguments.