Adds eigen dependency to Readme for OSX
Adds eigen dependency to Readme for OSX

This was necessary to get OpenGV to install on OSX Sierra for me.
sunbingfengPI/OpenSFM_Test,oscarlorentzon/OpenSfM,oscarlorentzon/OpenSfM,mapillary/OpenSfM,sunbingfengPI/OpenSFM_Test,mapillary/OpenSfM,BrookRoberts/OpenSfM,oscarlorentzon/OpenSfM,mapillary/OpenSfM,sunbingfengPI/OpenSFM_Test,oscarlorentzon/OpenSfM,BrookRoberts/OpenSfM,BrookRoberts/OpenSfM,BrookRoberts/OpenSfM,BrookRoberts/OpenSfM,sunbingfengPI/OpenSFM_Test,mapillary/OpenSfM,mapillary/OpenSfM,sunbingfengPI/OpenSFM_Test,oscarlorentzon/OpenSfM