Add PRDR docs.

diff --cc doc/doc-txt/experimental-spec.txt
index 7bb17883c77143434880f96872197045d72b3a00,7bb17883c77143434880f96872197045d72b3a00..e209ad83869556e0b52d05814b020e41212c81a3
--- 1/doc/doc-txt/experimental-spec.txt
--- 2/doc/doc-txt/experimental-spec.txt
+++ b/doc/doc-txt/experimental-spec.txt
@@@ -6,6 -6,6 +6,23 @@@ about experimental  features, all  of w
  liable to incompatible change.
  
  
++PRDR support
++--------------------------------------------------------------
++
++Per-Recipient Data Reponse is an SMTP extension proposed by Eric Hall
++in a (now-expired) IETF draft from 2007.  It's not hit mainstream
++use, but has apparently been implemented in the META1 MTA.
++
++ ref: http://www.eric-a-hall.com/specs/draft-hall-prdr-00.txt
++
++If Exim is built with EXPERIMENTAL_OCSP there is a new config
++boolean "prdr_enable" while controls whether PRDR is advertised
++as part of an EHLO response, a new "acl_data_smtp_prdr" ACL
++(called for each recipient, after data arrives but before the
++data ACL), and a new smtp transport option "hosts_try_prdr".
++
++
++
  OCSP Stapling support
  --------------------------------------------------------------