Add maintenance README and fix minor download bug

Neither of these changes requires that an existing maintenance page be
rebuilt.

The bug fix ensures that downloads are only attempted on strings found
within `url(...)` in *CSS* files, so that downloads do not start (and
fail) when `url(...)` is encountered in JavaScript and SVG files.
Merge pull request #1661 from openjck/maintenance-improvements

Add maintenance README and fix minor download bug