Add comment to docs when running `make route` target

[LimKianAn]

The nexthops change every time if we do a brand new make and if you want to add the new routes, it just tells you it already exists. Now, the description of this catch is added to README.

[GrigoriyMikhalkin]

IMO, too mouthful) I would write something like:

add this route to communicate with the virtual internet network 100.255.254.0/24 over leaf01 and leaf02. Route should be refreshed each time mini-lab is rebooted.

[Gerrit91]

I think with #61 the behavior changed a bit and the text needs to be adapted.

[Gerrit91]

If there's no progress going on in this issue, I'll close this in the near future.

[Gerrit91]

Thanks, @LimKianAn