Add note about strict types

Fixes #21.

Author: kelunik

README.md

@@ -74,6 +74,8 @@ Any implementation MUST at least provide these two parameters. The implementatio
 
 All registered callbacks MUST be executed in the order they were registered. If one of the callbacks throws an `Exception` or `Throwable`, it MUST be forwarded to `Async\Interop\Promise\ErrorHandler::notify`. The `Promise` implementation MUST then continue to call the remaining callbacks with the original parameters.
 
+Promise implementations MUST NOT use strict types (`declare(strict_types=1)`).
+
 ## Contributors
 
 * [Aaron Piotrowski](https://github.com/trowski)