Include physical dimensions in hardware overview

[Frijol]

To resolve this issue, carefully measure a Tessel 2 board with calipers (metric preferred), including the following:

• External dimensions of board with/without headers/ethernet jack/USB ports
• Mounting hole dimensions
• Mounting hole placement

List the dimensions in a new markdown file under "hardware" called "dimensions" or similar.

For bonus points, include a diagram.

For extra bonus points, make a 3D CAD model of the board and make it available e.g. through Thingiverse and link to it.

Possibly @nahog already knows these things after creating http://www.thingiverse.com/thing:2132746 ?

[Frijol]

Mounting holes are 2.5mm per https://forums.tessel.io/t/tessle-2-what-is-the-size-of-the-mounting-holes/3303

[Frijol]

In case helpful, the full parts list for the board is here:

https://docs.google.com/spreadsheets/d/1hgn9fT9eEDG23kPYqsrsC8mXqOHHuY68sc9VK9puuPo/edit?usp=sharing

In particular, the USB and Ethernet ports may be helpful:

https://www.digikey.com/product-detail/en/1001-004-01010/1175-1003-ND/3064733

https://www.digikey.com/products/en?x=0&y=0&lang=en&site=us&keywords=j0011D01BNL