Issue #189: Define in README.md the implemented PSR specifications.

[alexmerlin]

Mentioned more than just PSR-7 and/or PSR-15 in the readme file, because we are using all of the below standards:

• PSR-3: Logger Interface - the application uses LoggerInterface for error logging
• PSR-4: Autoloader - the application locates classes using an autoloader
• PSR-7: HTTP message interfaces - the handlers return ResponseInterface
• PSR-11: Container interface - the application is container-based
• PSR-15: HTTP Server Request Handlers - the handlers implement RequestHandlerInterface