MSI installer: offer to display the README file after install.

MSI installer: offer to display the README file after install.

This is a thing that the Inno Setup installer did, and that I didn't
get round to replicating when I rushed out the initial MSI in a hurry.

I've checked that this doesn't prevent unattended installation by
administrators: running 'msiexec /q /i putty-whatever.msi' as
administrator still installs silently after this change, without
popping up the README unexpectedly on anyone's desktop as a side
effect.

(I _think_ - but I'm still a long way from an MSI expert - that that's
because /q turns off the whole UI part of the MSI system, and the
loading of README is actually triggered by the transition away from
the final UI dialog box, which we now never visit in the first place.)