Merge pull request facebookresearch#770 from facebookresearch/readme_…

…update

Updated README to Clarify MacOS Installation

README.md

@@ -98,7 +98,8 @@ If you use the Habitat platform in your research, please cite the [Habitat](http
        conda install habitat-sim withbullet headless -c conda-forge -c aihabitat
       ```
 
-      See habitat-sim's [installation instructions](https://github.com/facebookresearch/habitat-sim#installation) for more detials.
+      See habitat-sim's [installation instructions](https://github.com/facebookresearch/habitat-sim#installation) for more details.
+      MacOS does *not* work with `headless` so exclude that argument if on MacOS.
 
 
 3. Run the example script `python examples/example.py ` which in the end should print out number of steps agent took inside an environment (eg: `Episode finished after 18 steps.`).