chore: update README

Changes requested in php-fig/http-factory#9

Author: gnumoksha

README.md

@@ -5,7 +5,7 @@ This repository holds all interfaces related to [PSR-7 (HTTP message interfaces)
 
 Note that this is not an HTTP Message implementation of its own. It is merely interfaces that describe the components of an HTTP Message.
 
-You can find [implementations][implementation-url] and [installation instructions][package-url] for the specification on the packagist.
+The installable [package][package-url] and [implementations][implementation-url] are listed on Packagist.
 
 [psr-url]: http://www.php-fig.org/psr/psr-7/
 [package-url]: https://packagist.org/packages/psr/http-message