Adds meshms markdown documentation

The configuration commands are already well documented in another Markdown file.  Please link to it, do not re-quote the `config schema` command here, it increases the maintenance load if the config commands are ever changed.  Instead, please quote the exact config options and give examples of their use.  See https://github.com/servalproject/serval-dna/blob/development/doc/Mesh-Packet-Filtering.md#how-to-configure-packet-filtering for an example.