Create "CONTRIBUTING.md" file

Author: henriquemoody

CONTRIBUTING.md

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+# Contributing to Respect\Rest
+
+Contributions to Respect\Rest are always welcome. You make our lives easier by
+sending us your contributions through GitHub pull requests.
+
+Pull requests for bug fixes must be based on the current stable branch whereas
+pull requests for new features must be based on `master`.
+
+Due to time constraints, we are not always able to respond as quickly as we
+would like. Please do not take delays personal and feel free to remind us here,
+on IRC, or on Gitter if you feel that we forgot to respond.
+
+## Using Respect\Rest From a Git Checkout
+
+The following commands can be used to perform the initial checkout of Respect\Rest:
+
+```shell
+git clone git://github.com/Respect/Rest.git
+cd Rest
+```
+
+Retrieve Respect\Rest's dependencies using [Composer](http://getcomposer.org/):
+
+```shell
+composer install
+```
+
+## Running Tests
+
+After run `composer install` on the library's root directory you must run PHPUnit.
+
+### Linux
+
+You can test the project using the commands:
+```shell
+$ vendor/bin/phpunit
+```
+
+### Windows
+
+You can test the project using the commands:
+```shell
+> vendor\bin\phpunit
+```
+
+No test should fail.
+
+You can tweak the PHPUnit's settings by copying `phpunit.xml.dist` to `phpunit.xml`
+and changing it according to your needs.
+
+## Standards
+
+We are trying to follow the [PHP-FIG](http://www.php-fig.org)'s standards, so
+when you send us a pull request, be sure you are following them.
+
+## Sending your code to us
+
+Please see http://help.github.com/pull-requests/.