Documentation for the -precert flag for "openssl req"

Reviewed-by: NTim Hudson <tjh@openssl.org>
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/843)

doc/man1/req.pod

@@ -37,6 +37,7 @@ B<openssl> B<req>
 [B<-newhdr>]
 [B<-extensions section>]
 [B<-reqexts section>]
+[B<-precert>]
 [B<-utf8>]
 [B<-nameopt>]
 [B<-reqopt>]
@@ -253,6 +254,14 @@ request extensions. This allows several different sections to
 be used in the same configuration file to specify requests for
 a variety of purposes.
 
+=item B<-precert>
+
+a poison extension will be added to the certificate, making it a
+"pre-certificate" (see RFC6962). This can be submitted to Certificate
+Transparency logs in order to obtain signed certificate timestamps (SCTs).
+These SCTs can then be embedded into the pre-certificate as an extension, before
+removing the poison and signing the certificate.
+
 =item B<-utf8>
 
 this option causes field values to be interpreted as UTF8 strings, by