Add PowerShell instructions for windows user

[cruxcode]

Add PowerShell instructions for windows user for following commands:

1. ASSET_URL_PREFIX=<YOUR_ORG_NAME> atri build ssg. The first part where it says ASSET_URL_PREFIX="..." works only in terminals like bash, zsh etc. For Powershell and cmd in windows, you have to first set the environment variable ASSET_URL_PREFIX by doing something like this in PowerShell env:ASSET_URL_PREFIX = 'YOUR_ORG_ NAME'
2. GIT_USER=<username> atri deploy ssg --gh-pages. Same as above.

[darshitac11]

I also want to discuss the installation steps for PowerShell users -