Add installation instructions

[alyssadai]

This program requires Python >= 3.0.

The program can be installed by:

1. Running the following command in a terminal: git clone https://github.com/CoBrALab/PyQC.git
2. Downloading the latest release file from here and extracting the contents locally.

Some examples of how you can run PyQC:

# Run PyQC to open images manually using the GUI
python PyQC-2.0/PyQC.py

# Load all images from a directory on the command line
python PyQC-2.0/PyQC.py PNC_CIVET_verify/*

# Load images via text file containing paths (one per line) to images of interest
python PyQC-2.0/PyQC.py $(cat PNC_CIVET_verify_list)