more images for doc

Rev: tutorial/circle50.pnm:1.1
Rev: tutorial/ryoki_carrot.png:1.1

.gitattributes

@@ -3,6 +3,7 @@
 *.db binary
 *.gif binary
 *.gz binary
+*.png binary
 *.pnm binary
 *.rs binary
 init_nt -text -crlf