Minor readme updates.
Minor readme updates.

Add link to define POD.
Refer to entity instead of component.
alecthomas/entityx,jaynus/entityx,arvidsson/entityx,arvidsson/entityx,drewet/entityx,asmodehn/entityx,Bablawn3d5/entityx,jarrettchisholm/entityx,asmodehn/entityx,asmodehn/entityx,drewet/entityx,alecthomas/entityx,Bablawn3d5/entityx,asmodehn/entityx,jaynus/entityx,jarrettchisholm/entityx,alecthomas/entityx,jarrettchisholm/entityx,drewet/entityx,arvidsson/entityx,Bablawn3d5/entityx,jaynus/entityx