docs: add a note on running isolated nodes — heartwood

Signed-off-by: Lars Wirzenius liw@liw.fi

 to prevent them from interfering with an existing installation of radicle. See the
 following section on environment variables.
 ## Running in isolation
 When testing things, if you want to run one or more nodes in isolation
 from the public Radicle network of nodes:
 * In the node `config.json` file:
   - set `preferredSeeds` to `[]`
   - set `node.connect` to `[]`
   - set `node.network` to `test`
   - set `node.peers.type` to `static`
 This allows you to make any changes to the repositories on your nodes
 without having them replicated to outside your nodes. This would be
 good to do for any experiments you make that might be burdensome for
 other nodes, such as creating very large repositories.
 ## Environment variables
 When developing radicle, some environment variables may be used to make the