readme: support experiment resource urls

A normal .pagespeed. url looks like:

    256x192vPuzzle.jpg.pagespeed.ic.alA0AdlR-2.jpg

If someone turns on RunExperiment, however, we generate urls like:

    256x192vPuzzle.jpg.pagespeed.a.ic.alA0AdlR-2.jpg
    256x192vPuzzle.jpg.pagespeed.b.ic.alA0AdlR-2.jpg

The new section is always a single letter between a and z.  We need to catch
these and pass them through to PageSpeed.