Set 2 soft spaces for contributions

Author: mynetx

CONTRIBUTING.md

@@ -3,7 +3,7 @@
 - New features are added to the develop branch, please base your feature-add pull requests on that branch.
 
 ### Code style
-- Please use 4 soft spaces per indent level.
+- Please use 2 soft spaces per indent level.
 - Take a look at the coding style used in codebird.php and apply the same convention to your contributed code.
 The styleguide is also available online at http://pear.php.net/manual/en/standards.php.