Adding suggested rationale by @lattner

Author: llvm-beanz

llvm/docs/DeveloperPolicy.rst

@@ -1154,13 +1154,16 @@ acceptable for their contributions.  We feel that a high burden for relicensing
 is good for the project, because contributors do not have to fear that their
 code will be used in a way with which they disagree.
 
-Embedded Copyright Statements
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+Embedded Copyright or 'Contributed by' Statements
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 The LLVM project does not accept contributions that include in-source copyright
 notices except where such notices are part of a larger external project being
 added as a vendored dependency.
 
+LLVM source code lives for a long time and is edited by many people, the best
+way to track contributions is through revision control history.
+
 Relicensing
 -----------