[WIP] Implement subroutine opcodes

Author: Peppece

docs/contributing.rst

@@ -6,8 +6,8 @@ Thank you for your interest in contributing! We welcome all contributions no mat
 Setting the stage
 ~~~~~~~~~~~~~~~~~
 
-.. note::
-    If you are on macOS and it is the first time you install Py-EVM, you will need to install LevelDB with brew:
+.. warning::
+    If you are on macOS and it is the first time you install Py-EVM, you will need to install LevelDB with brew before taking the following steps:
 
     .. code:: sh