Added Requires: header.

pep-0001.txt

@@ -184,6 +184,7 @@ PEP Style
       * Discussions-To: <email address>
         Status: <Draft | Active | Accepted | Deferred | Final | Replaced>
         Type: <Informational | Standards Track>
+      * Requires: <pep numbers>
         Created: <date created on, in dd-mmm-yyyy format>
       * Python-Version: <version number>
         Post-History: <dates of postings to python-list and python-dev>
@@ -201,6 +202,9 @@ PEP Style
     the author, or on the python-list or python-dev email mailing
     lists.
 
+    PEPs may have a Requires: header, indicating the PEP numbers that
+    this PEP depends on.
+
     PEPs may also have a Replaced-By: header indicating that a PEP has
     been rendered obsolete by a later document; the value is the
     number of the PEP that replaces the current document.  The newer