Improve README.md

Improve description, add **Installation**, **Usage** and **Issues**.

Author: DavidePastore

README.md

@@ -1,2 +1,42 @@
 # composer-audit
-A composer plugin that checks if your application uses dependencies with known security vulnerabilities (it uses SensioLabs Security Checker)
+A composer plugin that checks if your application uses dependencies with known security vulnerabilities (it uses [SensioLabs Security Checker][1]).
+
+## Installation
+Using the `composer` command:
+
+    $ composer require davidepastore/composer-audit:0.1.0
+
+Manually adding in composer.json:
+
+    {
+      "require": {
+        "davidepastore/composer-audit": "0.1.0"
+      }
+    }
+
+## Usage
+The checker will be executed when you launch `composer install` or `composer update`.
+If you have alerts in your composer.lock, `composer-audit` will print them. An example could be this:
+
+    ALERTS from SensioLabs security advisories.
+
+     *** dompdf/dompdf[v0.6.0] ***
+
+     * dompdf/dompdf/CVE-2014-2383.yaml
+    Arbitrary file read in dompdf
+    https://www.portcullis-security.com/security-research-and-downloads/security-advisories/cve-2014-2383/
+    CVE-2014-2383
+    
+    
+    Please fix these alerts from SensioLabs security advisories.
+
+If no alert is found, you'll get this:
+
+    All good from SensioLabs security advisories.
+
+## Issues
+
+If you have issues, just open one [here][2].
+
+[1]: https://github.com/sensiolabs/security-checker
+[2]: https://github.com/DavidePastore/composer-audit/issues