net/dns/resolvconffile: link to FAQ about resolv.conf being overwritten

Add link to new http://tailscale.com/s/resolvconf-overwrite page,
added in tailscale/tailscale-www#2243

Change-Id: I9718399487f2ed18bf1a112581fd168aea30f232
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/dns/direct_test.go

@@ -88,6 +88,7 @@ func testDirect(t *testing.T, fs wholeFileFS) {
 		t.Fatal(err)
 	}
 	want := `# resolv.conf(5) file generated by tailscale
+# For more info, see http://tailscale.com/s/resolvconf-overwrite
 # DO NOT EDIT THIS FILE BY HAND -- CHANGES WILL BE OVERWRITTEN
 
 nameserver 8.8.8.8

net/dns/resolvconffile/resolvconffile.go

@@ -42,6 +42,7 @@ type Config struct {
 func (c *Config) Write(w io.Writer) error {
 	buf := new(bytes.Buffer)
 	io.WriteString(buf, "# resolv.conf(5) file generated by tailscale\n")
+	io.WriteString(buf, "# For more info, see http://tailscale.com/s/resolvconf-overwrite\n")
 	io.WriteString(buf, "# DO NOT EDIT THIS FILE BY HAND -- CHANGES WILL BE OVERWRITTEN\n\n")
 	for _, ns := range c.Nameservers {
 		io.WriteString(buf, "nameserver ")