Add composer instructions to docs

[burzum]

https://github.com/CakeDC/recaptcha/blob/master/Docs/Documentation/Installation.md

Is missing the usual "composer require" thing most plugin docs have. ;)

[GeekWiinik]

execute in your proyect with CMD (command line)
composer require cakedc/recaptcha:@stable

[burzum]

@GeekWiinik I know what needs to be done, I'm saying this is missing from the docs but it's usually there.