doc: add go command notes to the Go 1 doc

Fixes #2912.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5783048

doc/go1.html

@@ -2087,7 +2087,25 @@ The semantic changes make it difficult for the fix tool to update automatically.
 <h2 id="cmd_go">The go command</h2>
 
 <p>
-TODO: Write this.
+Go 1 introduces the <a href="/cmd/go/">go command</a>, a tool for fetching,
+building, and installing Go packages and commands. The <code>go</code> command
+does away with makefiles, instead using Go source code to find dependencies and
+determine build conditions. Most existing Go programs will no longer require
+makefiles to be built.
+</p>
+
+<p>
+See <a href="/doc/code.html">How to Write Go Code</a> for a primer on the
+<code>go</code> command and the <a href="/cmd/go/">go command documentation</a>
+for the full details.
+</p>
+
+<p>
+<em>Updating</em>:
+Projects that depend on the Go project's old makefile-based build
+infrastructure (<code>Make.pkg</code>, <code>Make.cmd</code>, and so on) should
+switch to using the <code>go</code> command for building Go code and, if
+necessary, rewrite their makefiles to perform any auxiliary build tasks.
 </p>
 
 <h2 id="cmd_cgo">The cgo command</h2>

doc/go1.tmpl

@@ -1959,7 +1959,25 @@ The semantic changes make it difficult for the fix tool to update automatically.
 <h2 id="cmd_go">The go command</h2>
 
 <p>
-TODO: Write this.
+Go 1 introduces the <a href="/cmd/go/">go command</a>, a tool for fetching,
+building, and installing Go packages and commands. The <code>go</code> command
+does away with makefiles, instead using Go source code to find dependencies and
+determine build conditions. Most existing Go programs will no longer require
+makefiles to be built.
+</p>
+
+<p>
+See <a href="/doc/code.html">How to Write Go Code</a> for a primer on the
+<code>go</code> command and the <a href="/cmd/go/">go command documentation</a>
+for the full details.
+</p>
+
+<p>
+<em>Updating</em>:
+Projects that depend on the Go project's old makefile-based build
+infrastructure (<code>Make.pkg</code>, <code>Make.cmd</code>, and so on) should
+switch to using the <code>go</code> command for building Go code and, if
+necessary, rewrite their makefiles to perform any auxiliary build tasks.
 </p>
 
 <h2 id="cmd_cgo">The cgo command</h2>