Missing documentation for devs dealing with subgraphs.

[phazei]

There's documentation for a end user to make and use a subgraph, but not for devs on how to deal with them.

I noticed that the id's sent for items in the subgraph are "##:##" instead of just "##". I presume the first number is the subgraph number, and the second number is the node number. But when getting this.id we only get the node number. So we need documentation on all the intricacies of anything new we have to deal with regarding subgraphs.