Add migration guide for version 0.6.1 (#376).
Add migration guide for version 0.6.1 (#376).

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