add id tags to contribute.html

R=r
http://go/go-review/1025033

doc/contribute.html

 <!-- Contributing to the Go project -->
 
-<h2>Introduction</h2>
+<h2 id="Introduction">Introduction</h2>
 
 <p>
 This document explains how to write a new package,
@@ -21,7 +21,7 @@ enables discussions about design before much code
 has been written.
 </p>
 
-<h2>Community resources</h2>
+<h2 id="Community_resources">Community resources</h2>
 
 <p>
 For real-time help, there may be users or developers on
@@ -44,7 +44,7 @@ that receives a message summarizing each checkin to the Go repository.
 </p>
 
 
-<h2>Creating a new package</h2>
+<h2 id="New_package">Creating a new package</h2>
 
 <p>
 The source code for the package with import path
@@ -141,7 +141,7 @@ Writing clean, idiomatic Go code is beyond the scope of this document.
 that topic.
 </p>
 
-<h2>Testing</h2>
+<h2 id="Testing">Testing</h2>
 
 <p>
 Go has a lightweight test framework known as <code>gotest</code>.
@@ -193,7 +193,7 @@ Once your new code is tested and working,
 it's time to get it reviewed and submitted.
 </p>
 
-<h2>Code review</h2>
+<h2 id="Code_review">Code review</h2>
 
 <p>
 Changes to Go must be reviewed before they are submitted,