Added the sign up link for those that want to receive announcements of all new SAMPL related news (http://eepurl.com/dPj11j)
Added the sign up link for those that want to receive all announcements specific to SAMPL6 and the current log P prediction challenge (http://eepurl.com/gaAA0H).
Removed the link to the D3R page for the time being, as we don't want people signing up to the D3R mailing list. Can add back a link to the D3R website once the "Join our challenge" link gets updated (https://drugdesigndata.org/about/sampl6).
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