[~TASK] Moved doc to README

Author: David Fuhr

.gitignore

@@ -1,2 +1,4 @@
 /.idea
+/composer.phar
+/doc
 /vendor

README.md

@@ -80,7 +80,7 @@ How to set up your ```composer.json``` in your project:
 }
 ```
 
-## Testing ##
+## Testing
 
 First install dev-stuff:
 
@@ -89,3 +89,42 @@ First install dev-stuff:
 ```
 
 then run ```phpunit``` in projekt-root directory
+
+## How to Overwrite Dependencies
+
+We don't want to use always the official repositories for specific dependencies.
+For example for development purpose or use versions with custom patches.
+
+For this case you have the _repositories_ section inside your project composer.json
+Here you can define own package composer.json for specific dependencies by the package name.
+
+This example shows how to use a local git projects local-master branch which even works without a composer.json inside
+and a project in VCS with existing composer.json, which is not yet on packagist.
+
+```json
+"repositories": [
+   {
+      "type": "package",
+      "package": {
+         "name": "magento-hackathon/magento-composer-installer",
+         "version": "dev-master",
+         "type": "composer-installer",
+         "source": {
+            "type": "git",
+            "url": "/public_projects/magento/magento-composer-installer/",
+            "reference": "local-master"
+         },
+         "autoload": {
+            "psr-0": {"MagentoHackathon\\Composer\\Magento": "src/"}
+         },
+         "extra": {
+            "class": "\\MagentoHackathon\\Composer\\Magento\\Installer"
+         }
+      }
+   },
+   {
+      "type": "vcs",
+      "url": "git://github.com/firegento/firegento-germansetup.git"
+   }
+]
+```