Meta: Add note about new people making large architectural changes

Ideally, new contributors should hack on the system for a while before
attempting to make large architectural changes.

This will help ensure that architectural work is more in line with
the project direction, and that everyone's time is better spent.

CONTRIBUTING.md

@@ -55,3 +55,4 @@ Nobody is perfect, and sometimes we mess things up. That said, here are some goo
 * Use weasel-words like "refactor" or "fix" to avoid explaining what's being changed.
 * Include commented-out code.
 * Write in C.
+* Attempt large architectural changes until you are familiar with the system and have worked on it for a while.