ANNOUNCE: give the URL to the list of mirrors

ANNOUNCE: give the URL to the list of mirrors

The upstream FTP might be slow for some users. Further, the upstream
FTP was down for a brief period of time [1]. For more discussion on
adding the URL to the list of mirrors, see [2].

Thanks to Joel Kulesza for the idea.

[1]
https://www.mail-archive.com/search?l=mid&q=AM3PR04MB1297EE16DF1C3AA9FA54445CDEF60%40AM3PR04MB1297.eurprd04.prod.outlook.com
[2]
https://www.mail-archive.com/search?l=mid&q=CAAGOGw_PbNG4gGcmOg%2BgPG2%2Bm_Kot9bHCTf7GPYCNTOf_TrbOQ%40mail.gmail.com