Add aside about using CNAT space for overlay network

Author: jasikpark

docs/guides/tips-and-tricks/index.mdx

@@ -19,3 +19,5 @@ fail.
 Nebula is a [peer-to-peer](https://en.wikipedia.org/wiki/Peer-to-peer) VPN, meaning by default it only routes to hosts
 that have the software installed with an associated cert. If you want Nebula to function like
 [OpenVPN](https://en.wikipedia.org/wiki/OpenVPN), set up [unsafe routes](/docs/guides/unsafe_routes/).
+
+Consider using some/all of `100.64.0.0/10` as your overlay network, `its_free_real_estate.jpg`.