OMR specification and unofficial files

[Philippe Hurbain]

Following the problem mentionned here and after discussions with Mikeheide about MPDCenter, I think that OMR specifications should be updated to make it clear that unofficial parts MUST be included in OMR conform files.
It is more or less implied by this paragraph of the specification but it should be made much more explicit

Unofficial parts are allowed to be used. The filename of the unofficial part is subject to the following naming rules:

<Set Number>[-<Optional Qualifier>] - <Unofficial Part Number>.dat

Where:
<Set Number> is the the number printed on the model's container.
<Optional Qualifier> is a sequential number, starting with 1, added if there is more than one set that could be assigned <Set Number>.
<Unofficial Part Number> is the unofficial part number assigned in that very moment.

Example:
33956.dat would be renamed to 3345 - 33956.dat or 3345-1 - 33956.dat.