Update README.md

Added rule clarifications under "Risk of Catan's Art of War": Liberate, Annex, Siege, Lifting Siege. 
Deleted "One cannot move through roads that are controlled by other players unless negotiated." from "Army movement and Battle" section because of redundancy. 
Fixed headings to make format look "cleaner". Ex: Risk of Catan vs. Risk of catan
Added clarification under "Army Movement and Battle" about what hexes armies can move across.
Added Clarification under "Annexing Roads"