Revision 1837935

Remove obsolete references to SHA1.

* publish/docs/community-guide/releasing.part.html
    Don't refer to 'SHA1 checksums', just say 'checksums', as the kind we use
    has changed and may change again and isn't important here.

* publish/download.html
  (verifying): Don't mention SHA1. Only mention one commonly available program
    'sha512sum'. (GNU sha*sum programs are now part of 'coreutils'. Readers
    able to make use of that information don't need to be told it here.)