added some info on reporting issues and some rewording

CONTRIBUTING.md

@@ -5,17 +5,36 @@ Please note that this project is released with a
 [Contributor Code of Conduct](http://contributor-covenant.org/version/1/0/0/).
 By participating in this project you agree to abide by its terms.
 
+Reporting Issues
+----------------
+
+When reporting issues, please try to be a descriptive as possible, and include
+as much relevant information as you can. A step by step guide on how to
+reproduce the issue will greatly increase the chances of your issue being
+resolved in a timely manner.
+
+For example, if you are experiencing a problem while running one of the
+commands, please provide full output of said command in very very verbose mode
+(`-vvv`, e.g. `composer install -vvv`).
+
+If your issue involves installing, updating or resolving dependencies, the
+chance of us being able to reproduce your issue will be much higher if you
+share your `composer.json` with us.
+
 Installation from Source
 ------------------------
 
-Prior to contributing to Composer, you must use be able to run the tests.
+Prior to contributing to Composer, you must be able to run the test suite.
-To achieve this, you must use the sources and not the phar file.
+To achieve this, you need to acquire the Composer source code:
 
 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`
+You can run the test suite by executing `vendor/bin/phpunit` when inside the
+composer directory, and run Composer by executing the `bin/composer`. To test
+your modified Composer code against another project, run `php
+/path/to/composer/bin/composer` inside that project's directory.
 
 Contributing policy
 -------------------