add routeros to overlay

pkgs/routeros/README.md

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+# ppp-server
+
+To test a router, we need an upstream connection. This directory
+contains a derivation to download, start and configure a RouterOS
+"Cloud Hosted Router" instance in a Qemu VM. It is currently
+set up for automated tests only, and may require some manual
+frobbing to run interactively.
+
+Note that you need to open some multicast ports if you're using the
+NixOS firewall (or probably, any other firewall). For iptables you can
+accomplish this by editing your configuration.nix or some module it
+calls:
+
+
+```
+    networking.firewall.extraCommands = ''
+      ip46tables -A nixos-fw -m pkttype --pkt-type multicast -p udp --dport 1234:1236 -j nixos-fw-accept
+    '';
+```
+
+## Provenance
+
+The chr-7.x.img image is taken from https://mikrotik.com/download -
+look in the section titled "Cloud Hosted Router" for "Raw disk image".
+Note that this is proprietary software: please read the license
+information and make sure you're using it legally.