Update README.md

Add note on minimum RAM requirements

README.md

@@ -34,3 +34,5 @@ ls /etc/rc.d/S*tailscale*
 5. Reboot the router and verify that it shows up online on the [Tailscale Admin portal](https://login.tailscale.com/admin/machines).
 
 6. To update the version of tailscale, grab the latest version [here](https://pkgs.tailscale.com/stable/#static) of the form `1.2.10_mips` and replace the same in `/usr/bin/tailscale` and `/usr/bin/tailscaled`: `version="1.2.10_mips"`.
+
+Note: You need to have atleast 11+16 = ~27 MB of free space in `/tmp` (which is usually in RAM) to be able to use this.