Add to man page: time format for 'valid'

Created by: robertbruntz

The man page for version 0.7.1 reports:

       -v VALID, -valid VALID, --valid VALID
              time requirement for proxy to be valid

It might be helpful for users to know what format(s) "VALID" can take - e.g., arbitrary number of seconds, HOURS:MINUTES, DAYS:HOURS:MINUTES:SECONDS, etc. Note that the homologous line in grid-proxy-info was:

-valid HOURS:MINUTES, -v HOURS:MINUTES, -hours HOURS, -h HOURS
           Check that the proxy certificate is valid for at least HOURS hours and MINUTES minutes. If it is not, grid-proxy-info will exit with exit code 1.

but ecp-cert-info does not have the self-documenting -hours or -h options, which breaks drop-in compatibility.

(Unrelated: PyPI page has that same page as the Homepage. Just pointing that out in case that was unintentional, e.g., if the GitHub page should have been the home page.