message: Add README section for using custom connect accounts
diff: @@ -63,5 +63,19 @@ to determine which fields are required for either request.
  let params = stripe::ChargeListParams::default();
  let charges = stripe::Charge::list(&client, params).unwrap();
  - println!("{}", charges.data.len()); // => 1
  + println!("{:?}", charges); // => List { data: [Charge { id: "ch_12345", .. }] }
  +```
  +
  +### Using Custom Connect accounts
  +This crate supports impersonating a custom connect account.
  +
  +To impersonate the account get a new Client and pass in the account id.
  +
  +```rust
  + let client = client.with(stripe::Params{stripe_account: Some("acct_ABC")});
  +
  + // Then, all requests can be made normally
  + let params = stripe::CustomerListParams::default();
  + let customers = stripe::Customer::list(&client, params).unwrap();
  + println!("{:?}", customers); // => List { data: [Customer { .. }] }
  ```
  