Add MEP-004: Removal of ProtoBuf from the protocol

> Why have zero-prefixed number at all then? Which machine will read MEPs?

As mentioned, mainly for filesystem ordering.

> 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.

The index is created by enumerating the document files in lexicographic order, and since the filenames are "mep-001.md" etc., the list in the index will be ordered too.

> I already told @lclc to add the zero prefix. Can we change it once we have MEP0-5 finished?

I don't see the point of doing so. Let's reach an agreement and follow it.