Updates features.

  Enable to omit import statement of standard libraries,
  why applied using goimports in #576bf15.

Signed-off-by: Kouhei Maeda <mkouhei@palmtb.net>
Added omit import statement of system global installed packages to roadmap.

Signed-off-by: Kouhei Maeda <mkouhei@palmtb.net>
Added Known issues.

Signed-off-by: Kouhei Maeda <mkouhei@palmtb.net>