Documentation: rng_seed

Just discovered this by searching for 'rng' in the repo.

I tried to generate reproducible networks in https://github.com/acteng/netgen/pull/22/commits/f31cf84dfd4d6eccd9b8eb3e968302a3a53fb35d but, having re-run

```bash
sudo docker run -v $(pwd):/app ghcr.io/urban-analytics-technology-platform/od2net:main /app/config.json
```

and comparing the results with

```bash
diff /tmp/output.geojson output/output.geojson
```

After copying the previous version it seems there are still differences.

Probably worth documenting and happy to give it a go, after I figure it out myself.