Add readme and image

Author: tschie

src/main/java/com/esri/samples/tiledlayers/export_vector_tiles/ExportVectorTiles.png

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+<h1>Export Vector Tiles</h1>
+
+<p>Demonstrates how to export vector tiles with its style from a vector tiled layer.</p>
+
+<p><img src="ExportVectorTiles.png"/></p>
+
+<h2>How to use the sample</h2>
+
+<p>First, a dialog will appear prompting for authentication credentials to arcgis.com. You must supply valid 
+credentials for an organization user to continue. When the vector tiled layer loads, zoom in to the extent you want 
+to export. The red box shows the extent that will be exported. Click the "Export Vector Tiles" 
+button to start the job. A progress indicator will show. The larger the extent, the longer it will take to export. When 
+finished, a dialog will show the exported result in a new map view.</p>
+
+<h2>How it works</h2>
+
+<p>To export tiles from an <code>ArcGISVectorTiledLayer</code>:</p>
+<ol>
+  <li>Create an <code>ExportVectorTilesTask</code>, passing in the <code>PortalItem</code> for the vector tiled layer. 
+  Since vector tiled layers are premium content, you must first authenticate with the Portal.</li>
+  <li>Create default <code>ExportTilesParameters</code> with <code>task.createDefaultExportTilesParametersAsync(extent, maxScale)</code>.</li>
+  <li>Call <code>task.exportVectorTilesAsync(defaultParams, vtpkPath, resourcePath)</code> to create the 
+  <code>ExportVectorTilesJob</code>. The resource path is required if you want to export the tiles with the style.</li>
+  <li>Call <code>job.start()</code> to start the export job.</li>
+  <li>When the job is done, use <code>job.getResult()</code> to get the resulting 
+  <code>ExportVectorTilesResult</code>.</li>
+  <li>You can load the result as a <code>ArcGISVectorTiledLayer</code> with <code>new ArcGISVectorTiledLayer(result.getVectorTileCache(), result.getItemResourceCache())</code>.</li>
+</ol>
+
+<h2>Features</h2>
+
+<ul>
+  <li>ArcGISVectorTiledLayer</li>
+  <li>ExportVectorTilesJob</li>
+  <li>ExportVectorTilesParamters</li>
+  <li>ExportVectorTilesResult</li>
+  <li>ExportVectorTilesTask</li>
+  <li>ItemResourceCache</li>
+  <li>Portal</li>
+  <li>PortalItem</li>
+  <li>UserCredential</li>
+  <li>VectorTileCache</li>
+</ul>