add install instructions for Mac to readme.md

[wpetry]

The current installation instructions are difficult to implement on Mac. I found that these steps worked for me on macOS 10.14.6:

1. Close QGIS.
2. Navigate to the symbolic link to Python3 within the QGIS application at /Applications/QGIS.app/Contents/MacOS/bin/python3. This will open a Terminal window with a message about the version of Python is running and will end with the interactive prompt >>>.
3. Enter the command import os
4. Install the libraries needed for Pic2Map:

• os.system('/Applications/QGIS.app/Contents/MacOS/bin/python3 -m pip install pyOpenGL')
• os.system('/Applications/QGIS.app/Contents/MacOS/bin/python3 -m pip install pyOpenGL_accelerate')
• os.system('/Applications/QGIS.app/Contents/MacOS/bin/python3 -m pip install piexif')

5. Enter the command exit(), then close the Terminal.
6. Reopen QGIS and download the Pic2Map plugin through the usual plugin manager (be sure to enable visibility of experimental plugins first).