CoC: Clarify which project we're talking about.

* CODE-OF-CONDUCT: Define "project" and "project maintainer" as
suggested by rms.

CODE-OF-CONDUCT

@@ -1,5 +1,8 @@
 Contributor Code of Conduct
 
+  Note: In the sequel, "project" refers to GNU Guix, and "project
+  maintainer(s)" refers to maintainer(s) of GNU Guix.
+
 As contributors and maintainers of this project, and in the interest of
 fostering an open and welcoming community, we pledge to respect all
 people who contribute through reporting issues, posting feature