Add information about client installation

[spyl94]

php-http/client-implementation makes sure we install a client otherwise composer installation fails.

[Nyholm]

Thank you. That is great.
What do you think, should we link to the documentation instead of the repositories?

http://docs.php-http.org/en/latest/clients/guzzle6-adapter.html
http://docs.php-http.org/en/latest/clients/curl-client.html

[xabbuh]

Or simply link to http://docs.php-http.org/en/latest/clients.html with the list of available adapters?

[spyl94]

Done!

[dbu]

thanks!