Add CONTRIBUTING.md with instructions on how to add a new test

CONTRIBUTING.md

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+Filing an Issue
+---------------
+
+If you've found something in a WordPress theme or plugin that should be reported by VIP-scanner, please file an issue with some sample code.
+
+Adding a New Check
+------------------
+
+Adding a new VIP-scanner check can be considered a testboook example of Test-Driven Development (TDD). Mind the difference between a test (as in TDD) and a check, which is a sort of code sniff specific to VIP-scanner, used for scanning a theme or plugin for bad code. We'll a start by a test that will ensure the functionality of our check.
+
+Here's a recipe:
+
+1. Think of a suitable name for your check. In this example, which is a simplified version of a check that actually ships with vip-scanner, we're going to add a check that scans files for ill-formed escaping, i.e. `esc_attr` being wrapped around `print` or `printf` statements instead of the other way round. We'll name our check class `EscapingCheck`.
+2. Create a file to hold some erroneous code in vip-scanner/tests/data. The file should be named like the check, only with `Check` replaced by `Test`; also, use `inc` instead of `php` for the file extension -- so in our example, EscapingTest.inc.
+3. Put the following code into that file:
+
+	<?php
+
+	die(); //Don't actually run the following code.
+
+4. Add the erroneous code the check should scan for after that:
+
+	<?php
+
+	die(); //Don't actually run the following code.
+
+	esc_attr( printf( 'unescaped string' ) );
+	esc_attr( print 'unescaped string' );
+
+5. Create a test file to cover your check in vip-scanner/tests/checks/. Name it like your check, prepended with 'test-': test-EscapingCheck.php. At the top of the file, put `require_once( 'CodeCheckTestBase.php' )` -- this file contains the `CodeCheckTestBase` class which is basically a wrapper around PHPUnit which adds some nifty extra features. Create a class that `extends CodeCheckTestBase`, and name it like your check, but ending in `Test` instead of `Check` (this is actually important for CodeCheckTestBase to work). For each different type of error your check is going to scan for, add a method to that class. As with PHPUnit, those methods need to be prefixed with `test` and use methods like `$this->assertEqual()` to make assertions for which PHPUnit checks.
+
+6. Each test method contains of three parts:
+
+	1. An array of expected errors
+	2. The actual check, which returns an array of actual errors found in your 'inc' file.
+	3. An assertion comparing those two arrays.
+
+7. At this point, you should put some thought into finding a slug and description for the error you're diagnosing, and what error level to assign to it -- the latter can be blocker, warning, and note. Use that data in the `$expected_errors` array like so: 
+
+	<?php
+
+	require_once( 'CodeCheckTestBase.php' );
+
+	class EscapingTest extends CodeCheckTestBase {
+
+		public function testEscaping() {
+			$expected_errors = array(
+				array( 'slug' => 'functions-file',
+					'level' => BaseScanner::LEVEL_BLOCKER,
+					'description' => sprintf(
+						__( 'The function %1$s is being passed as the first parameter of %2$s. This is problematic because %1$s echoes a string which will not be escaped by %2$s.', 'vip-scanner' ),
+						'<code>printf()</code>',
+						'<code>esc_attr()</code>'
+					),
+					'file' => 'EscapingTest.inc',
+					'lines' => 5,
+				),
+				array( 'slug' => 'functions-file',
+						'level' => BaseScanner::LEVEL_BLOCKER,
+						'description' => sprintf(
+								__( '%1$s is being passed as the first parameter of %2$s.', 'vip-scanner' ),
+								'<code>print</code>',
+								'<code>esc_attr()</code>'
+						),
+						'file' => 'EscapingTest.inc',
+						'lines' => 6,
+				),
+			);
+			$actual_errors = $this->checkFile( 'EscapingTest.inc' );
+			$this->assertEqualErrors( $expected_errors, $actual_errors );
+		}
+	}
+
+The 'file' and 'lines' values state where the check is supposed to find the erroneous code -- compare that to the EscapingTest.inc sample code above.
+
+7. From the top-level vip-scanner directory, run `phpunit`. Your test should fail because no class named `EscapingCheck` exists yet and thus can be found by `CodeCheckTestBase` -- not because of some syntax errors in your test file!
+
+8. Create your check file in vip-scanner/vip-scanner/checks/. In our case, that's `EscapingCheck.php`. Start with an explanatory commentary and the following stub:
+
+	<?php
+	/**
+	 * Check for ill-formed escaping, i.e. `esc_attr` being wrapped around
+	 * `print` or `printf` statements instead of the other way round.
+	 */
+
+	class EscapingCheck extends CodeCheck {
+		function __construct() {
+			parent::__construct( array(
+				// ...
+			) );
+		}
+	}
+
+TO BE CONTINUED.