Updated readme: include links to expression syntax document and generated API documentation; link build badge to GitHub Actions instead of Travis CI.
Updated readme: include links to expression syntax document and generated API documentation; link build badge to GitHub Actions instead of Travis CI.

radarsat1/libmapper,libmapper/libmapper,malloch/libmapper,libmapper/libmapper,radarsat1/libmapper,radarsat1/libmapper,malloch/libmapper,libmapper/libmapper,libmapper/libmapper,radarsat1/libmapper,malloch/libmapper,malloch/libmapper,libmapper/libmapper,libmapper/libmapper,radarsat1/libmapper,malloch/libmapper,malloch/libmapper