cmd/compile: minor updates to the README

Use an HTML comment with triple dashes for the copypright header, which
means that the paragraph will be ignored when rendering both HTML and
TeX.

While at it, quote "GC", and properly link to internal/ssa/README.md.

Change-Id: Ib18529d2fc777d836e74726ff1cfe685e08b063c
Reviewed-on: https://go-review.googlesource.com/109875Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/README.md

+<!---
 // Copyright 2018 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.
+-->
 
 ## Introduction to the Go compiler
 
@@ -19,7 +21,7 @@ the `go/*` packages were developed to enable writing tools working with Go code,
 such as `gofmt` and `vet`.
 
 It should be clarified that the name "gc" stands for "Go compiler", and has
-little to do with uppercase GC, which stands for garbage collection.
+little to do with uppercase "GC", which stands for garbage collection.
 
 ### 1. Parsing
 
@@ -113,4 +115,4 @@ and debugging information.
 ### Further reading
 
 To dig deeper into how the SSA package works, including its passes and rules,
-head to `cmd/compile/internal/ssa/README.md`.
+head to [cmd/compile/internal/ssa/README.md](internal/ssa/README.md).