http2/h2demo: update README

Fixes golang/go#34729

Change-Id: I8a5dde21d9e0f0a8b95500cc3bbbbca693aa1637
Reviewed-on: https://go-review.googlesource.com/c/net/+/199579
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

http2/h2demo/README

@@ -1,16 +1,19 @@
+This is a demo webserver that shows off Go's HTTP/2 support.
 
-Client:
- -- Firefox nightly with about:config network.http.spdy.enabled.http2draft set true
- -- Chrome: go to chrome://flags/#enable-spdy4, save and restart (button at bottom)
+It runs at https://http2.golang.org/ so people can hit our
+implementation with their HTTP/2 clients, etc. We intentionally do not
+run it behind any other HTTP implementation so clients (including
+people demonstrating attacks, etc) can hit our server directly. It
+just runs behind a TCP load balancer.
+
+When running locally, you'll need to click through TLS cert warnings.
+The dev cert was initially made like:
 
 Make CA:
 $ openssl genrsa -out rootCA.key 2048
 $ openssl req -x509 -new -nodes -key rootCA.key -days 1024 -out rootCA.pem
-... install that to Firefox
 
 Make cert:
 $ openssl genrsa -out server.key 2048
 $ openssl req -new -key server.key -out server.csr
 $ openssl x509 -req -in server.csr -CA rootCA.pem -CAkey rootCA.key -CAcreateserial -out server.crt -days 500
-
-