[Doc] Added a note to install composer globally

doc/00-intro.md

@@ -46,6 +46,8 @@ any version beginning with `1.0`.
 
 ### Downloading the Composer Executable
 
+#### Locally
+
 To actually get Composer, we need to do two things. The first one is installing
 Composer (again, this mean downloading it into your project):
 
@@ -61,10 +63,19 @@ option and providing a target directory (it can be an absolute or relative path)
 
     $ curl -s http://getcomposer.org/installer | php -- --install-dir=bin
 
+#### Globally
+
 You can place this file anywhere you wish. If you put it in your `PATH`,
 you can access it globally. On unixy systems you can even make it
 executable and invoke it without `php`.
 
+You can run these commands to easily acces `composer` from anywhere on your system:
+
+    $ curl -s http://getcomposer.org/installer | php
+    $ sudo mv composer.phar /usr/local/bin/composer
+
+Then, just run `composer` in order to run composer
+
 ### Using Composer
 
 Next, run the `install` command to resolve and download dependencies:

doc/03-cli.md

@@ -177,6 +177,11 @@ command. It will replace your `composer.phar` with the latest version.
 
     $ php composer.phar self-update
 
+If you have installed composer for your entire system (see [global installation](00-intro.md#globally)),
+you have to run the command with `root` privileges
+
+    $ sudo composer self-update
+
 ## create-project
 
 You can use Composer to create new projects from an existing package.