Add MEP-004: Removal of ProtoBuf from the protocol

Why have zero-prefixed number at all then? Which machine will read MEPs?

I can see it having a purpose when the index is created automatically form the title tag, so that `MEP-010` comes after `MEP-002`. When you drop the leading zeros this is not guaranteed.

I already told @lclc to add the zero prefix. Can we change it once we have MEP0-5 finished?