Add docs on how and why to use alternative weights.
Add docs on how and why to use alternative weights.

HealthyDelivery/vanity,assaf/vanity,peterkovacs/vanity,assaf/vanity,peterkovacs/vanity,Futurelearn/vanity,assaf/vanity,Futurelearn/vanity,peterkovacs/vanity,Futurelearn/vanity,HealthyDelivery/vanity,HealthyDelivery/vanity