doc: README: Add GitHub Actions status badge
doc: README: Add GitHub Actions status badge

It's good to have a CI status badge at the top of README to show off
how we are doing.

Signed-off-by: Yasushi SHOJI <afa9ca3be149447780c2f7b524d61ed3d3db63f9@spacecubics.com>

libcsp/libcsp,libcsp/libcsp