add README.md
fix the handling of overlays (gentoo bug #376775)

do not even try to open a non-existent directories
Turn \t into space.

The set builtin of bash reports tabs contained in variables as $'...\t...'
and up till now, strip_env would remove the ‘\’ but leave the ‘t’, causing
extra letters in e.g. the list of overlay directories.

See also https://bugs.gentoo.org/show_bug.cgi?id=526192
Merge pull request #16 from gagern/tabs

Turn \t into space.
Merge pull request #14 from bor/fix/overlays_handling

fix the handling of overlays (gentoo bug #376775)
Merge pull request #10 from bor/feature/readme

add README.md