Add slackhq/Nebula VPN to list of VPN software

README.md

@@ -643,6 +643,7 @@ Comparison of NoSQL servers: http://kkovacs.eu/cassandra-vs-mongodb-vs-couchdb-v
 * [strongSwan](https://www.strongswan.org/) - Complete IPsec implementation for Linux.
 * [tinc](http://www.tinc-vpn.org/) - Distributed p2p VPN.
 * [WireGuard](https://www.wireguard.com/) - Very fast VPN based on elliptic curve and public key crypto.
+* [Nebula](https://github.com/slackhq/nebula) - A scalable p2p VPN with a focus on performance, simplicity and security.
 
 ## Web