Windows instructions in "Validating Manifest"

[CookiePLMonster]

Currently, Validating Manifest instructions are only for POSIX systems, for Windows the steps are a bit different depending on whether you use cmd.exe or PowerShell.

I can PR those instructions in since I had to figure it out yesterday, but how would that be best structured? The two ideas I had were:

1. Have the instructions copied over twice, so each platform's instructions are "full":

   Validating Manifest

   POSIX systems

   You can check your manifest file for validity before submitting it to this repository. To do so, set up a virtual Python environment with the required dependencies:

   python3 -m venv venv
   source venv/bin/activate
   pip install -r tools/requirements.txt

   [and so on...]

   Windows (cmd)

   You can check your manifest file for validity before submitting it to this repository. To do so, set up a virtual Python environment with the required dependencies:

   python -m venv venv
   venv\Scripts\activate.bat
   python -m pip install -r tools\requirements.txt

   [and so on...]

   Windows (PowerShell)

   You can check your manifest file for validity before submitting it to this repository. To do so, set up a virtual Python environment with the required dependencies:

   python -m venv venv
   .\venv\Scripts\Activate.ps1
   python -m pip install -r tools\requirements.txt

   [and so on...]

2. Have full instructions only in the first sub-section, and then go for shorthands in Windows sections:

   Validating Manifest

   POSIX systems

   You can check your manifest file for validity before submitting it to this repository. To do so, set up a virtual Python environment with the required dependencies:

   python3 -m venv venv
   source venv/bin/activate
   pip install -r tools/requirements.txt

   [and so on...]

   Windows (cmd)

   Set up a virtual Python environment:

   python -m venv venv
   venv\Scripts\activate.bat
   python -m pip install -r tools\requirements.txt

   [and so on...]

   Windows (PowerShell)

   Set up a virtual Python environment:

   python -m venv venv
   .\venv\Scripts\Activate.ps1
   python -m pip install -r tools\requirements.txt

   [and so on...]

[doomwastaken]

Thank you for submission, I will ask hedger for his input, since he is original author of documentation