github-19: add instructions for pulling from github source

Author: ArturMoczulski

README.md

@@ -10,6 +10,17 @@ Rollbar error monitoring integration for Laravel projects. This library adds a l
 Installation
 ------------
 
+This package has not been submitted to Packagist yet. Until then you need to add the Github repository to your composer.json explicitly:
+```json
+    "repositories": [
+        {
+            "type": "vcs",
+            "url" : "https://github.com/rollbar/rollbar-php-laravel.git"
+        }
+    ]
+```
+This will change once the package makes its' way to Packagist.
+
 Install using composer:
 
 ```