Mention install-doc in README and CHANGES

Signed-off-by: Frank Terbeck <ft@bewatermyfriend.org>
Fixes in RELEASE-CHECKLIST

Signed-off-by: Frank Terbeck <ft@bewatermyfriend.org>
install-doc: Ignore non-existent files

Signed-off-by: Frank Terbeck <ft@bewatermyfriend.org>
Release version 0.5 of guile-termios

Signed-off-by: Frank Terbeck <ft@bewatermyfriend.org>