[RFC] Pre-vote notice for Nullable Types

The [RFC for Nullable Types][1] is going to go into the voting phase soon. There have been a few changes to the RFC in the meantime: - More example for documentation's sake - The vote is now split into two parts: one for nullable parameter types and one for nullable return types. - Updated BC break section. This RFC now contains the fix for [bug 71428][2] that was previously committed. [More information][3] can be found in the RFC. - Coauthored with Dmitry Stogov. Unless there are significant concerns raised this will go into voting phase early next week. [1]: https://wiki.php.net/rfc/nullable_types [2]: https://bugs.php.net/bug.php?id=71428 [3]: https://wiki.php.net/rfc/nullable_types#to_backward_compatibility