Checklist updates for pre-release setup.

I don't know why I never bothered to write it down before, but it's a
good idea to let a pre-release build actually *happen* between turning
them on and updating the website to claim they exist.

Also, for the first time ever, I've just sent out an announcement
email for the 0.78 *pre-releases*, soliciting testing in advance of
the actual release. So, add that to the checklist as well.

(cherry picked from commit 1a3655013d)

CHECKLST.txt

@@ -19,11 +19,17 @@ pre-releases on the website:
  - Edit ~/adm/puttysnap.sh on the master machine to enable pre-release
    builds, by changing the 'if false' to 'if true'.
 
+ - Wait for a nightly build to run, so that the first pre-release
+   snapshot actually exists.
+
  - Put the website into pre-release mode, by defining prerel_version()
    in components/Base.mc to return the upcoming version number. Also
    add a news announcement in components/news. (Previous naming
    convention has been to name it in the form 'X.YZ-pre.mi'.)
 
+ - Optionally: write an announcement email for the availability of
+   pre-releases, and send it out to <putty-announce@lists.tartarus.org>.
+
 Things to do during the branch-stabilisation period:
 
  - Go through the source (including the documentation), and the