Document .bazelignore in the user manual

Now that .bazelignore is functional (with 8570ee2 making
those directories fully ignored, even if containing symlink
cycles, and 6f8e36b fixing #5974), we can document this file.

Change-Id: Ib8ea7253c317b6c0ebee53f1d1c57c787a0b401b
PiperOrigin-RevId: 212996459

site/docs/user-manual.html

@@ -287,6 +287,21 @@ <h5>Example</h5>
   build:memcheck --strip=never --test_timeout=3600
 </pre>
 
+
+<h3 id='startup files'>
+  Other files governing Bazel's behavior
+</h3>
+
+<h4 id='.bazelignore'><code>.bazelignore</code>
+<p>
+  You can specify directories within the workspace
+  that you want Bazel to ignore, such as related projects
+  that use other build systems. Place a file called
+  <code>.bazelignore</code> at the root of the workspace
+  and add the directories you want Bazel to ignore, one per
+  line. Entries are relative to the workspace root.
+</p>
+
 <h2 id='build'>Building programs with Bazel</h2>
 <h3>The <code>build</code> command</h3>