Add Readme explaining how to use extjs5 for dev

www/manager5/Readme.md

+pveproxy with ExtJS 5 developpement mini howto
+==============================================
+
+unpack the ExtJS 5 sources, and copy them to /usr/share/pve-manager/ext5
+
+    cd www/ext5/
+    make install
+
+symlink the to our ext5 compatible javascript code
+
+    cd /usr/share/pve-manager
+    ln -s PATH_TO_YOUR_GIT_REPO/www/manager5
+
+access the PVE proxy with ExtJS 5
+
+    https://localhost:8006/?ext5=1
+
+
+With the extra parameter **ext5=1**, pve-proxy will call the function **PVE::ExtJSIndex5::get_index()**
+which returns a HTML page, with all javascript files included.
+Provided you included the javascript in **PVE/ExtJSIndex5.pm**, a simple browser refresh is then enough 
+to see your changes.