Migrate docs from source repo

[sdwheeler]

Migrate end-user content from the following files to the docs repository:

• https://github.com/PowerShell/PSScriptAnalyzer/blob/master/PowerShellBestPractices.md
• https://github.com/PowerShell/PSScriptAnalyzer/blob/master/README.md
• https://github.com/PowerShell/PSScriptAnalyzer/blob/master/ScriptRuleDocumentation.md