[PATCH] NEWS: add overview section for 0.18

[David Bremner]

This mentions the things I (subjectively) thought were most likely to
either annoy people or make it worth upgrading.
---
 NEWS | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/NEWS b/NEWS
index 06876e0..132c687 100644
--- a/NEWS
+++ b/NEWS
@@ -1,6 +1,19 @@
 Notmuch 0.18~rc0 (2014-04-22)
 =============================
 
+Overview
+--------
+
+This new release of includes some enhancements to searching for
+messages by filesystem location (`folder:` and `path:` prefixes under
+*General* below).  Saved searches in *Emacs* have also been enhanced
+to allow distinct search orders for each one.  Another enhancement to
+the *Emacs* interface is that replies to encrypted messages are now
+encrypted, reducing the risk of unintentional information disclosure.
+The default dump output format has changed to the more robust
+`batch-tag` format. The previously deprecated parsing of single
+message mboxes is removed. For detailed release notes, see below.
+
 General
 -------
 
-- 
1.9.2