This PR updates the design document and adds a readme document. It also fixes bugs:
a. Fix PowerShell API use to be compatible with Windows PowerShell 5.1.
b. Fix Get-Secret to detect correctly PSObject/string type.
c. Fix Get-Secret to always convert SecureString to plain text when -AsPlainText is selected.
This PR updates the design document and adds a readme document. It also fixes bugs: a. Fix PowerShell API use to be compatible with Windows PowerShell 5.1. b. Fix Get-Secret to detect correctly PSObject/string type. c. Fix Get-Secret to always convert SecureString to plain text when -AsPlainText is selected.