NEWS: add news for 0.18.1

[David Bremner]

Gregor Zattler <telegraph at gmx.net> writes:

> 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."
>

Good idea; I put in a slightly more terse version of that, and Tomi's
punctation changes, and pushed.

d