Add information about UnRARLib

[viktor02]

PS D:\WordSmith-master> python -V
Python 3.9.4
PS D:\WordSmith-master> python -m venv venv
PS D:\WordSmith-master> .\venv\Scripts\activate

(venv) PS D:\WordSmith-master> python -m pip install -r .\requirements.txt
Collecting unrar
  Using cached unrar-0.4-py3-none-any.whl (25 kB)
Installing collected packages: unrar
Successfully installed unrar-0.4
WARNING: You are using pip version 20.2.3; however, version 21.1 is available.
You should consider upgrading via the 'D:\WordSmith-master\venv\Scripts\python.exe -m pip install --upgrade pip' command.

(venv) PS D:\WordSmith-master> python .\WordSmith.py
Traceback (most recent call last):
  File "D:\WordSmith-master\WordSmith.py", line 3, in <module>
    from views import DisclaimerPage, HelpPage, MainPage, NavBar
  File "D:\WordSmith-master\views.py", line 4, in <module>
    from controller import Controller
  File "D:\WordSmith-master\controller.py", line 7, in <module>
    from unrar import rarfile
  File "D:\WordSmith-master\venv\lib\site-packages\unrar\rarfile.py", line 26, in <module>
    from unrar import unrarlib
  File "D:\WordSmith-master\venv\lib\site-packages\unrar\unrarlib.py", line 57, in <module>
    raise LookupError("Couldn't find path to unrar library.")
LookupError: Couldn't find path to unrar library.

I fixed this error after reading this answer on SO, but it seems to me that this information should be added to the Installation section in the README