Add build instructions for Windows

[kernel-panic96]

Please add build instructions for Windows, the direct pip install command from the README fails to install.
I think you have mentioned in another issue (issue comment) that it requires Microsoft Visual C++ Build Tools. Maybe a link to it and telling users explicitly that they need to install it, will be beneficial.

[jarviscodes]

The referenced issue should have been fixed in a previous release.
Are you getting the exact same error ?

If not, would you mind dumping a trace? I'll try to simulate and contribute to the readme.
For me, things work fine on Windows, apart from the fact that it's missing the wmi dependancy at runtime.

(venv) C:\Users\Cedric\PycharmProjects\testspec>pip3 install pyspectator
Collecting pyspectator
  Using cached pyspectator-1.2.2-py3-none-any.whl (13 kB)
Collecting enum34>=1.0
  Using cached enum34-1.1.10-py3-none-any.whl (11 kB)
Collecting netifaces>=0.10.4
  Using cached netifaces-0.10.9-cp37-cp37m-win_amd64.whl (16 kB)
Collecting psutil>=2.1.1
  Using cached psutil-5.7.3-cp37-cp37m-win_amd64.whl (243 kB)
Collecting pyvalid>=0.9
  Using cached pyvalid-1.0.4-py3-none-any.whl (11 kB)
Collecting six~=1.15
  Using cached six-1.15.0-py2.py3-none-any.whl (10 kB)
Installing collected packages: six, pyvalid, psutil, netifaces, enum34, pyspectator
Successfully installed enum34-1.1.10 netifaces-0.10.9 psutil-5.7.3 pyspectator-1.2.2 pyvalid-1.0.4 six-1.15.0

I should note that this is a dev machine and I do have VC++ installed for multiple other projects.