Improve subgraph / query documentation based on current practices

[trentmc]

Now that many things have stabilized, we can go back to our README documentation and improve our queries.

A useful input: from Alex in Slack:

I'm using this query for general overview on subgraph:

query{
  predictContracts{
    id
    token{
      name
    }
    subscriptions(orderBy: expireTime orderDirection:desc first:10){
      user {
        id
      }
      expireTime
    }
    slots(orderBy: slot orderDirection:desc first:10){
      slot
      roundSumStakesUp
      roundSumStakes
      predictions(orderBy: timestamp orderDirection:desc){
        stake
        user {
          id
        }
        timestamp
        payout{
          payout
          predictedValue
          trueValue
        }
      }
      trueValues{
        trueValue
      }
    }
  } 
}