docs: Added recommendation to use jCenter dependency for Volley

<li>Automatic scheduling of network requests.</li>

<li>Multiple concurrent network connections.</li>

<li>Transparent disk and memory response caching with standard HTTP

<a href=http://en.wikipedia.org/wiki/Cache_coherence">cache coherence</a>.</li>

<a href="http://en.wikipedia.org/wiki/Cache_coherence">cache coherence</a>.</li>

<li>Support for request prioritization.</li>

<li>Cancellation request API. You can cancel a single request, or you can set blocks or

scopes of requests to cancel.</li>

<a href="https://android.googlesource.com/platform/frameworks/volley">AOSP</a>

repository at {@code frameworks/volley} and contains the main request dispatch pipeline

as well as a set of commonly applicable utilities, available in the Volley "toolbox." The

easiest way to add Volley to your project is to clone the Volley repository and set it as

a library project:</p>

easiest way to add Volley to your project is to add the following dependency to your app's

build.gradle file:

<pre class="no-pretty-print">

dependencies {

    ...

    compile 'com.android.volley:volley:1.0.0'

}

</pre>

You can also clone the Volley repository and set it as a library project:</p>

<ol>

<li>Git clone the repository by typing the following at the command line:

<pre>

<pre class="no-pretty-print">

git clone https://android.googlesource.com/platform/frameworks/volley

</pre>

</li>