Update README.md

Author: jsiebens

README.md

@@ -1,5 +1,7 @@
 # SPIRE Tailscale Plugin
 
+> :warning:  this project is still WIP and experimental (see [#1](https://github.com/jsiebens/spire-tailscale-plugin/issues/1))
+ 
 This repository contains agent and server plugins for [SPIRE](https://github.com/spiffe/spire) to allow [Tailscale](https://tailscale.com) node attestation.
 
 ## Quick Start
@@ -38,4 +40,4 @@ The plugin operates as follows:
 1. Agent sends the key to the server
 1. Server inspects the key and checks if it is a valid key in its Tailscale network.
 1. Server creates a SPIFFE ID in the form of `spiffe://<trust_domain>/spire/agent/ts/<hostname>`
-1. All done!
+1. All done!