Update contributing guide

CONTRIBUTING.md

@@ -21,6 +21,10 @@ We accept contributions via Pull Requests on [Github](https://github.com/thephpl
 
 - **Send coherent history** - Make sure each individual commit in your pull request is meaningful. If you had to make multiple intermediate commits while developing, please squash them before submitting.
 
+- **Ensure tests pass!** - Please run the tests (see below) before submitting your pull request, and make sure they pass. We won't accept a patch until all tests pass.
+
+- **Ensure no coding standards violations** - Please run PHP Code Sniffer using the PSR-2 standard (see below) before submitting your pull request. A violation will cause the build to fail, so please make sure there are no violations. We can't accept a patch if the build fails.
+
 
 ## Running Tests
 
@@ -29,4 +33,10 @@ $ phpunit
 ```
 
 
+## Running PHP Code Sniffer
+
+``` bash
+$ ./vendor/bin/phpcs src --standard=psr2 -sp
+```
+
 **Happy coding**!