Add doc about customizing 'ext/lib' (#443).
Add doc about customizing 'ext/lib' (#443).

akirakw/asakusafw,asakusafw/asakusafw,cocoatomo/asakusafw,ashigeru/asakusafw,akirakw/asakusafw,ashigeru/asakusafw,asakusafw/asakusafw,cocoatomo/asakusafw