Updated README
Updated README

Use `Unix` instead of `Linux` in badge `alt` attribute
(e.g. we have macOS in the matrix).

LibSass and SassC don't have the submodule relation to be accurate.
We manually clone `sassc` repo in `libsass` directory, vice versa or
set `SASS_LIBSASS_PATH` environment variable (`LIBSASS_DIR` in case of
Windows) when they do not share the common root on file system.
am11/libsass,am11/libsass,am11/libsass,am11/libsass,am11/libsass