Clarified required implementation package

Folowup to #53

Author: GrahamCampbell

README.md

@@ -25,7 +25,7 @@ Feel free to check out the [change log](CHANGELOG.md), [releases](https://github
 
 This version requires [PHP](https://php.net) 7.2-7.4.
 
-To get the latest version, simply require the project using [Composer](https://getcomposer.org). You will need to install any package that "provides" `psr/http-client-implementation`. Pure PHP users will want something like:
+To get the latest version, simply require the project using [Composer](https://getcomposer.org). You will need to install package that "provide" [`psr/http-client-implementation`](https://packagist.org/providers/psr/http-client-implementation) and [`psr/http-factory-implementation`](https://packagist.org/providers/psr/http-factory-implementation). Pure PHP users will want something like:
 
 ```bash
 $ composer require bitbucket/client:^3.0 guzzlehttp/guzzle:^7.0.1 http-interop/http-factory-guzzle:^1.0

composer.json

@@ -18,6 +18,7 @@
         "php-http/multipart-stream-builder": "^1.1",
         "psr/cache": "^1.0",
         "psr/http-client-implementation": "^1.0",
+        "psr/http-factory-implementation": "^1.0",
         "psr/http-message": "^1.0",
         "symfony/polyfill-php80": "^1.17"
     },