README.md: add coverity scan build status
README.md: add coverity scan build status

Signed-off-by: Michael Ira Krufky <00524723a60798c74a43fcc620c25dd7b9ece078@linuxtv.org>
mkrufky/libdvbtee,mkrufky/libdvbtee,mkrufky/libdvbtee,mkrufky/libdvbtee,mkrufky/libdvbtee