Initialize .gitattributes for doc

.gitattributes instructs git custom diff handling which helps the
readability of git diff in development. This is especially useful for
binary files such as images, as it prevents git from printing
meaningless diffs.

Differential Revision:	https://reviews.freebsd.org/D43082

.gitattributes

@@ -0,0 +1,9 @@
+*.css   diff=css
+*.html  diff=html
+*.pl    diff=perl
+*.rb    diff=ruby
+
+# Images are binary files
+*.ico   binary
+*.jpg   binary
+*.png   binary