Note about adding permisions to binary (#3936)

in regards to binding to lower ports than 1024

contrib/systemd/gitea.service

@@ -22,6 +22,11 @@ WorkingDirectory=/home/git/gitea
 ExecStart=/home/git/gitea/gitea web
 Restart=always
 Environment=USER=git HOME=/home/git
+# If you want to bind Gitea to a port below 1024 uncomment
+# the two values below
+###
+#CapabilityBoundingSet=CAP_NET_BIND_SERVICE
+#AmbientCapabilities=CAP_NET_BIND_SERVICE
 
 [Install]
 WantedBy=multi-user.target