Update upgrade guide

Author: barryvdh

UPGRADE.md

@@ -6,16 +6,18 @@
 The primary difference is the HTTP Client. We are now using HTTPlug (http://httplug.io/) but rely on our own interface.
 
 ### Breaking
-- Change typehint from Guzzle ClientInterface to `Omnipay\Common\Http\Client`
-- `$client->get('..')`/`$client->post('..')` will send the request directly, no longer need to call `->send()`.
-- Instead of `$client->createRequest(..)->send()`, you should call `$client->sendRequest($client->createRequest(..))`.
-- When sending an AJAX body, convert the body to a string with `json_encode`.
+- Change typehint from Guzzle ClientInterface to `Omnipay\Common\Http\ClientInterface`
+- `$client->get('..')`/`$client->post('..')` etc are removed, you can call `$client->request('GET', '')`.
+- No need to call `$request->send()`, requests are sent directly.
+- Instead of `$client->createRequest(..)` you can create+send the request directly with `$client->request(..)`.
+- When sending a JSON body, convert the body to a string with `json_encode()` and set the correct Content-Type.
 - The response is a PSR-7 Response object. You can call `$response->getBody()->getContents()` to get the body as string.
-- `$response->json()` and `$response->xml()` are gone, but `Omnipay\Common\Http\ResponseParser::json($response)` 
-and  `Omnipay\Common\Http\ResponseParser::xml($response)` can be used instead.
+- `$response->json()` and `$response->xml()` are gone, but you can implement the logic directly.
 - An HTTP Client is no longer added by default by `omnipay/common`, but `omnipay/omnipay` will add Guzzle. 
 Gateways should not rely on Guzzle or other clients directly.
-
-### Deprecated
+- `$body` should be a string (eg. `http_build_query($data)` or `json_encode($data)` instead of just `$data`).
 - The `$headers` parameters should be an `array` (not `null`, but can be empty)
-- `$body` should be a string (eg. `http_build_query($data)` or `json_encode($data)` instead of just $data.
+
+Note: Prior to stable release, the goal is to migrate to PSR-18, once completed. 
+This will not change the gateway implementations.
+