Update index.rst

Fixed the broken link on 'automated build system' - now it points to the counterparty.io website 'Build from source'. This files is synced with http://counterparty.io/docs/counterpartyd/, the link should probably point to  https://github.com/CounterpartyXCP/counterpartyd_build/blob/master/docs/BuildingFromSource.rst but not sure how to sync that with the site, so I just pointed it directly to the site.