add instructions to change Android SDK location when using N Preview

<p>To download Android Studio 2.1 as a separate installation, follow these

steps (or if you want to receive Android Studio 2.1 as an update to your

existing installation, skip to step 4):</p>

existing installation, skip to step 6):</p>

<ol>

  <li>Edit the name of your

  </li>

  <li>Unzip the package and move the Android Studio 2.1 contents to the

    appropriate location for applications on your system, then launch it.</li>

  <li>In the setup wizard, click <strong>Next</strong> a couple times to

  reach the SDK Components Setup screen. Here,

  you must set a different path for the <strong>Android SDK Location</strong>.

  It can be any directory you want, but it must not be the default path, which

  is the same directory used by your original version of Android Studio (unless

  you changed that one yourself). The SDK location for Android Studio 2.1

  must be different because the N Preview SDK packages are not compatible with

  versions of Android Studio lower than 2.1.</p>

  </li>

  <li>Continue with the setup wizard until you reach the Android Studio IDE.</li>

  <li>Open the Settings dialog

    (<strong>File &gt; Settings</strong> on Windows/Linux, or

    <strong>Android Studio &gt; Preferences</strong> on Mac). In the left