v0.4.0

• Retry writing images and indexes in unpkg command
• Improve error message when image cannot be retrieved during packaging
• Improve packaging docs

6aa3deefd419daf93bc0f9a4c36f9f5a087b3412ca676b2fce6e42b02d863150  ./kbld-darwin-amd64
7a4c44ebeb410cbee51bba272c2843835d071c6b01b230a53299a0dbabefd1f9  ./kbld-linux-amd64
ef8cc0e9912eaece281f6736942f7d5636f728e779831f3ce9ac02defe9c1e56  ./kbld-windows-amd64.exe