Added more content to README
Added more content to README

Explanation of subgroups and shards, guide to constructing a Group, and
guide to using ordered_send, ordered_query, p2p_send, and p2p_query.

Derecho-Project/derecho-unified,Derecho-Project/derecho-unified,Derecho-Project/derecho-unified,Derecho-Project/derecho-unified,Derecho-Project/derecho-unified