HACKING: Document how to add standard copyright claims to files

* (REQUIRED READING) Copyright claims may be documented in the standard way,
   with a "Copyright ..." line near the beginning of the file.

Incorporating suggestions and refinements from:

Lou Berger <lberger@labn.net>
diff --git a/HACKING.md b/HACKING.md
index f926ed6..3bd1eb3 100644
--- a/HACKING.md
+++ b/HACKING.md
@@ -50,6 +50,22 @@
 _not_ sufficient.  The "Signed-off-by" line is not used by the Quagga
 project.
 
+You may document applicable copyright claims to files being modified or
+added by your contribution.  For new files, the standard way is to add a
+string in the following format near the beginning of the file:
+
+    Copyright (C) <Year> <name of person/entity>[, optional contact details]
+
+When adding copyright claims for modifications to an existing file, please
+preface the claim with "Portions: " on a line before it and indent the
+"Copyright ..." string. If such a case already exists, add your indented
+claim immediately after. E.g.:
+
+    Portions:
+      Copyright (C) <Year> <Entity A> ....
+      Copyright (C) <Year> <Your details> [optional brief change description]
+
+
 GUIDELINES FOR HACKING ON QUAGGA {#sec:guidelines}
 ================================