Add a README and basic usage documentation

[jcelerier]

The repository has no README, wiki or documentation.

For a tool distributed in the Suite SAT it needs at least:

• What it does: real-time translation of spatialization OSC messages between SpatGRIS (/spat/serv), ADM-OSC v1.0 and SPAT Revolution.
• Supported protocols in/out and the default port (18032).
• How to run it and where to download the builds (releases page).
• A short note on the convention handling (azimuth sign, degrees/radians, per-source state, index offsets).