Clarify what this file is used for.

(And also "who does what", even if we can't say that in a way the computers understand.)

Change-Id: Ieaed05ecb241d764335f27db97fe382e6e674eee
diff --git a/janitors/OWNERS b/janitors/OWNERS
index c25d9e4..b317151 100644
--- a/janitors/OWNERS
+++ b/janitors/OWNERS
@@ -1,7 +1,19 @@
-# OWNERS file for projects that don't really have owners so much as volunteer janitors.
+# go/android-3p requires that all external projects have the "janitors" in
+# their OWNERS files.
+
+# These are also the "owners" for projects that don't really have owners
+# so much as volunteer janitors.
+
+# General maintenance.
+sadafebrahimi@google.com
+
+# C/C++.
 ccross@google.com
 cferris@google.com
-dwillemsen@google.com
 enh@google.com
+
+# Java.
 maco@google.com
-sadafebrahimi@google.com
+
+# Python.
+dwillemsen@google.com