[PATCH] doc: document notmuch-dump header line

[David Bremner <david@tethera.net>]

This was introduced with the libconfig changes, but not documented then.
---

after some IRC discussion with Tomi, this rst is slightly less gross

 doc/man1/notmuch-dump.rst | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/doc/man1/notmuch-dump.rst b/doc/man1/notmuch-dump.rst
index eda9e07..94986a8 100644
--- a/doc/man1/notmuch-dump.rst
+++ b/doc/man1/notmuch-dump.rst
@@ -86,7 +86,15 @@ Supported options for **dump** include
 	Output per-message metadata, namely tags. See *format* above
 	for description of the output.
 
-      The default is to include both tags and configuration information
+      The default is to include both tags and configuration
+      information. As of version 2 of the dump format, there is a
+      header line of the following form
+
+      |
+      |  #notmuch-dump <*format*>:<*version*> <*included*>
+
+      where <*included*> is a comma separated list of the above
+      options.
 
     ``--output=``\ <filename>
         Write output to given file instead of stdout.
-- 
2.8.1