Add attachment bracket STL / freecad files

[Dantali0n]

Hello I have created a tiny simple bracket to hold the adapters in place, Figured you mind want to add them to assets and refer to them in the README.

I can create a pull request that does this if that's easier for you but didn't as this gives you the opportunity to use your own file structure and wording.

bracket-hp-366-flr-network-card.zip

Screw holes are loosely sized so no threading as that is taken care of by the holes it will bind to themselves. Its aimed as a one size fits all solution rather than creating a very tight tolerance.