move contributing instructions to a separate file

A link to the CONTRIBUTING.md will show up on Github's pull request
creation form. Plus, this makes the README shorter and more relevant for
users.

CONTRIBUTING.md

@@ -0,0 +1,29 @@
+Contributing to Composer
+========================
+
+Installation from Source
+------------------------
+
+Prior to contributing to Composer, you must use be able to run the tests.
+To achieve this, you must use the sources and not the phar file.
+
+1. Run `git clone https://github.com/composer/composer.git`
+2. Download the [`composer.phar`](https://getcomposer.org/composer.phar) executable
+3. Run Composer to get the dependencies: `cd composer && php ../composer.phar install`
+
+You can now run Composer by executing the `bin/composer` script: `php /path/to/composer/bin/composer`
+
+Contributing policy
+-------------------
+
+All code contributions - including those of people having commit access -
+must go through a pull request and approved by a core developer before being
+merged. This is to ensure proper review of all the code.
+
+Fork the project, create a feature branch, and send us a pull request.
+
+To ensure a consistent code base, you should make sure the code follows
+the [Coding Standards](http://symfony.com/doc/current/contributing/code/standards.html)
+which we borrowed from Symfony.
+
+If you would like to help, take a look at the [list of issues](http://github.com/composer/composer/issues).

README.md

@@ -32,18 +32,6 @@ themselves. To create libraries/packages please read the
 3. Run Composer: `php composer.phar install`
 4. Browse for more packages on [Packagist](https://packagist.org).
 
-Installation from Source
-------------------------
-
-To run tests, or develop Composer itself, you must use the sources and not the phar
-file as described above.
-
-1. Run `git clone https://github.com/composer/composer.git`
-2. Download the [`composer.phar`](https://getcomposer.org/composer.phar) executable
-3. Run Composer to get the dependencies: `cd composer && php ../composer.phar install`
-
-You can now run Composer by executing the `bin/composer` script: `php /path/to/composer/bin/composer`
-
 Global installation of Composer (manual)
 ----------------------------------------
 
@@ -55,20 +43,6 @@ Updating Composer
 Running `php composer.phar self-update` or equivalent will update a phar
 install with the latest version.
 
-Contributing
-------------
-
-All code contributions - including those of people having commit access -
-must go through a pull request and approved by a core developer before being
-merged. This is to ensure proper review of all the code.
-
-Fork the project, create a feature branch, and send us a pull request.
-
-To ensure a consistent code base, you should make sure the code follows
-the [Coding Standards](http://symfony.com/doc/current/contributing/code/standards.html)
-which we borrowed from Symfony.
-
-If you would like to help take a look at the [list of issues](http://github.com/composer/composer/issues).
 
 Community
 ---------