nmbug: Update clone instructions for v0.18

With notmuch v0.18 the nmbug repository format changed and we gained a
clone command [1,2].  Add a link to the live nmbug version, so it's
easier for folks to download nmbug independently from notmuch.

[1]: c200167 (nmbug: Add 'clone' and replace FETCH_HEAD with
     @{upstream}, 2014-03-09)
[2]: 512ffe3 (NEWS: Document the recent 'nmbug clone' and @{upstream}
     changes, 2014-04-15)