Added paragraph about tools being allowed to modify default extras behavior

Author: astrofrog

peps/pep-0771.rst

@@ -233,6 +233,14 @@ recognized as invalid in a dependency specification, it should be ignored, and
 if all specified extras are invalid, then this should be considered equivalent
 to ``package[]`` (rather than ``package``) and *not* install any default extras.
 
+Finally, we note (as also discussed in `Relying on tooling to deselect
+any default extras`_) that package installers are allowed to implement their
+own options to control the above behavior, for example implementing an option
+that disables default extras for some or all packages regardless of where these
+packages appear in the dependency tree -- although if such options are
+implemented, they should be opt-in, and users should experience the above
+behavior as default.
+
 Examples
 --------