Added a README section about contributing and the commands you can ru…

…n to test out your code.

README.markdown

@@ -32,6 +32,20 @@ The documentation for PHP\_CodeSniffer is available in the [PEAR manual](http://
 
 Information about upcoming features and releases is available on the [Squiz Labs blog](http://www.squizlabs.com/php-codesniffer).
 
+Contributing
+-------------
+
+If you do contribute code to PHP\_CodeSniffer, please make sure it conforms to the PEAR coding standard and that the PHP\_CodeSniffer unit tests still pass. The easiest way to contribute is to work on a checkout of the repository, or your own fork, rather than an installed PEAR version. If you do this, you can run the following commands to check if everything is ready to submit:
+
+    cd PHP_CodeSniffer
+    php scripts/phpcs --ignore=*/tests/* . -n
+
+Which should give you no output, indicating that there are no PEAR coding standard errors. And then:
+
+    phpunit tests/AllTests.php
+
+Which should give you no failures or errors. You can ignore any skipped tests as these are for external tools.
+
 Issues
 ------