Update INSTALL.md

Added a quick note for installing pySerial on macOS

Author: redsnout

INSTALL.md

@@ -128,6 +128,10 @@ Then install `pySerial`:
 ```Bash
 $ pip install pyserial & pip2 install pyserial
 ```
+To install pySerial on OS X: Download pyserial from [PyPI](https://pypi.org/project/pyserial/#files), unpackage the download, and CD into the folder. Then run the following command:
+```Bash
+$ sudo python3 setup.py install
+```
 
 ### 4. Copy and edit a `Makefile`