Add a documentation for pdfjs sidebar auto-opening. See #895

Author: cdujeu

core/src/plugins/editor.pdfjs/plugin_doc.html

@@ -1,2 +1,6 @@
 <p>PDF.JS JavaScript PDF viewer from Mozilla beautifully integrated into Pydio.</p>
-<p align="center"><img src="frame_image.png" alt="PDF Viewer"></p>
+<p align="center"><img src="editor-pdfjs.png.png" alt="PDF Viewer"></p>
+<p>If you want to have the viewer automatically open its sidebar with pages thumbnails, you have to manually apply this modification:
+    <br>line 407 in "plugins/editor.pdfjs/pdfjs/web/viewer.js", change 0 to 1 like this :
+    <br>sidebarViewOnLoad: 1,
+</p>