cmd/relui: add package comment

Copy the description of relui to a package comment.
Adjust README formatting so the header shows up as expected.

For golang/go#40279.

Change-Id: I74b9ee7ff67c94c885aea5231393928e8b5be647
Reviewed-on: https://go-review.googlesource.com/c/build/+/245645
Reviewed-by: Alexander Rakoczy <alex@golang.org>

cmd/relui/README.md

@@ -1,4 +1,4 @@
-#golang.org/x/build/cmd/relui
+# golang.org/x/build/cmd/relui
 
 ```
                ▀▀█             ▀
@@ -9,4 +9,3 @@
 ```
 
 relui is a web interface for managing the release process of Go.
-

cmd/relui/main.go

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// relui is a web interface for managing the release process of Go.
 package main
 
 import (

cmd/relui/protos/protos.go

@@ -1,6 +1,7 @@
 // Copyright 2020 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
+
 package protos
 
 // Run "go generate" in this directory to update. You need to have: