Add static images used by getFeedTree

TTRSS' getFeedTree operation references the URLs of icons included with the software. These icons are from the Silk icon set (see README in this commit), which is compatible with our license, so we simply use them as well.

www/tt-rss/images/README

@@ -0,0 +1,21 @@
+Silk icon set v1.3 
+Copyright 2006, Mark James
+http://www.famfamfam.com/lab/icons/silk/
+
+Used under license:
+http://creativecommons.org/licenses/by/2.5/
+
+A minimal subset of the Silk icon set used by Tiny Tiny RSS is included here 
+to provide consistent results with certain API functions. 
+
+Note that TT-RSS renames some of the icons, and we use the modified names, 
+again for consistency. Below is a table listing the source file names:
+
+Modified        Original
+-----------     --------------
+archive.png     box.png
+feed.png        feed.png
+folder.png      folder.png
+fresh.png       cup.png
+label.png       tag_yellow.png
+time.png        time.png