README.md: add license & build status badges
README.md: add license & build status badges

A bit of repo "branding".

It also highlights if anything is broken with the
master branch.

Signed-off-by: Alexandru Ardelean <c8ec8d7216643db8a88f8dd26a8db4e136d951e0@analog.com>

analogdevicesinc/libad9361-iio,analogdevicesinc/libad9361-iio,analogdevicesinc/libad9361-iio,analogdevicesinc/libad9361-iio