Update documentation to use `virtualenv`
Update documentation to use `virtualenv`

README file now suggests using `virtualenv` to create isolated `python` environment to manage installed dependencies separately from system-level packages.
mapillary/OpenSfM,mapillary/OpenSfM,sunbingfengPI/OpenSFM_Test,oscarlorentzon/OpenSfM,sunbingfengPI/OpenSFM_Test,oscarlorentzon/OpenSfM,sunbingfengPI/OpenSFM_Test,mapillary/OpenSfM,BrookRoberts/OpenSfM,sunbingfengPI/OpenSFM_Test,BrookRoberts/OpenSfM,BrookRoberts/OpenSfM,sunbingfengPI/OpenSFM_Test,mapillary/OpenSfM,BrookRoberts/OpenSfM,oscarlorentzon/OpenSfM,BrookRoberts/OpenSfM,oscarlorentzon/OpenSfM,mapillary/OpenSfM,oscarlorentzon/OpenSfM