Quick installation guide
Step 1
Move the contents of the .zip to your /Scripts/ folder in Reaper. For Windows this can be: C:\Users\<username>\AppData\Roaming\REAPER\Scripts
Step 2
Import the ReaChorder.py into the action window (new/load).
Then you can load it in the MIDI piano-roll editor, or add script action to the toolbar as a shortcut. (right click on the toolbar in the MIDI editor). It must be called from the MIDI piano-roll editor.