[WIP] Implement subroutine opcodes

Author: Peppece

docs/contributing.rst

@@ -6,6 +6,15 @@ 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:
+
+    .. code:: sh
+        brew install python3 LevelDB
+
+
+..
+
 First we need to clone the Py-EVM repository. Py-EVM depends on a submodule of the common tests across all clients, so we need to clone the repo with the ``--recursive`` flag. Example:
 
 .. code:: sh