Add doc and link to it in the options page

The doc describes all the features that the extension
provides. This change also adds a link to this doc in the options page
so it can be accessed quickly.

Fixes: #23
Change-Id: Iad1d54ba216d6dd4e7820fe2a2ed8644f86ed945
diff --git a/docs/resources/compress_gif.bash b/docs/resources/compress_gif.bash
new file mode 100644
index 0000000..01cb969
--- /dev/null
+++ b/docs/resources/compress_gif.bash
@@ -0,0 +1,6 @@
+# Helper tool used to compress generated GIFs
+gifsicle -O3 --lossy=100 --colors 128 $1 -o $2
+# webp isn't yet allowed to be used in gitiles
+#gif2webp -mixed -min_size -loop_compatibility $2