[WIP] Implement subroutine opcodes

Author: Peppece

docs/contributing.rst

@@ -6,9 +6,7 @@ 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 before taking the following steps:
+**Only on macOS:** if it is the first time you install Py-EVM, you will need to install LevelDB with brew before taking the following steps:
 
 .. code:: sh