add info about enabling USB debugging to adb doc.

<div id="qv">

  <h2>In this document</h2>

<ol>

  <li><a href="#Enabling">Enabling adb Debugging</a></li>

  <li><a href="#issuingcommands">Syntax</a></li>

  <li><a href="#commandsummary">Commands</a></li>

  <li><a href="#devicestatus">Querying for Emulator/Device Instances</a></li>

instance from any client (or from a script).</p>

<h2 id="Enabling">Enabling adb Debugging</h2>

<p>In order to use adb with a device connected over USB, you must enable

<strong>USB debugging</strong> in the device system settings, under <strong>

Developer options</strong>.</p>

<p>On Android 4.2 and higher, the Developer options screen is

hidden by default. To make it visible, go to

<b>Settings &gt; About phone</b> and tap <b>Build number</b> seven times. Return to the previous

screen to find <strong>Developer options</strong> at the bottom.</p>

<p>On some devices, the Developer options screen may be located or named differently.</p>

<p class="note"><strong>Note:</strong> When you connect a device running Android 4.2.2 or higher

to your computer, the system shows a dialog asking whether to accept an RSA key that allows

debugging through this computer. This security mechanism protects user devices because it ensures

SDK Platform-tools r16.0.1 and higher) in order to debug on a device running Android 4.2.2 or

higher.</p>

<p>For more information about connecting to a device over USB, read

<a href="{@docRoot}tools/device.html">Using Hardware Devices</a>.</p>

<h2 id="issuingcommands">Syntax</h2>