Updated README

Author: ondrejmirtes

README.md

@@ -22,7 +22,8 @@
 * Require parameter and return typehints for functions and methods (either native or phpDocs)
 * Correct case for referenced and called function names.
 * Correct case for inherited and implemented method names.
-* Contravariance for parameter types and covariance for return types in inherited methods
+* Contravariance for parameter types and covariance for return types in inherited methods (also known as Liskov substitution principle - LSP)
+* Check LSP even for static methods
 
 Additional rules are coming in subsequent releases!