tailcfg: note RegionID reservation
@@ -48,6 +48,9 @@ type DERPRegion struct {
//
// RegionIDs must be non-zero, positive, and guaranteed to fit
// in a JavaScript number.
+ //
+ // RegionIDs in range 900-999 are reserved for end users to run their
+ // own DERP nodes.
RegionID int
// RegionCode is a short name for the region. It's usually a popular
