Add initial README file

[tgorochowik]

Describe the goal of the project.

The README will need to be kept up to date as the project progresses.

[pgielda]

I would also strongly recommend adding a LICENSE file from the very beginning. Please use Apache 2.0. (this file https://github.com/antmicro/scalenode/blob/master/LICENSE)

[pkatarzynski]

Please add a block diagram for the board to README to indicate what interfaces are you going to implement