Merge pull request #2 from php-http/liskov

Added note about Liskov

proposed/http-client/http-client.md

@@ -15,6 +15,10 @@ The goal of this PSR is to allow developers to create libraries decoupled from H
 implementations. This would make libraries more stable since the reduced number of 
 dependencies and the likelihood to get in version conflicts is reduced.
 
+The second goal is that all HTTP clients should follow the [Liskov substitutions principle][Liskov]. 
+This means that all clients should act the same when sending a request. By default a HTTP client 
+should not follow redirect nor throw exceptions on HTTP responses with status 4xx or 5xx. 
+
 ## Interfaces
 
 The following interfaces MAY be implemented together within a single class or
@@ -144,6 +148,4 @@ interface HttpException extends Exception
   public function getResponse();
 }
 ```
-
-
-
+[Liskov]: https://en.wikipedia.org/wiki/Liskov_substitution_principle