NEWS: add news for 0.18.1

[Gregor Zattler]

Hi David, notmuch developers,
* David Bremner <david at tethera.net> [21. Jun. 2014]:
> This is my summary of the commits from 0.18.

[...]
> +Fix for phrase indexing
> +
> +  There were several bugs involving overlapping "phrases" accross
> +  header and mime part boundaries.

Perhaps this should be amended with the following excerpt from a
commit message:

"This fix only affects newly indexed messages.  Messages that are
already indexed won't benefit from this fix without re-indexing, but
the fix won't make things any worse for existing messages."


Ciao; Gregor