Add reference to Wiki in USAGE.md (#298)

Add a reference to the documentation in the Wiki to the top of USAGE.md.

Author: HowardWolosky

USAGE.md

@@ -2,6 +2,7 @@
 ## Usage
 
 #### Table of Contents
+*   [Full Module Documentation](#full-module-documentation)
 *   [Logging](#logging)
 *   [Telemetry](#telemetry)
 *   [Common PowerShell API Patterns](#common-powershell-api-patterns)
@@ -130,6 +131,20 @@
 
 ----------
 
+## Full Module Documentation
+
+All commands for the module have "Comment-Based Help" available at your fingertips.
+You can access that help at any time by running:
+
+```powershell
+Get-Help -Full <commandName>
+```
+
+In addition to accessing it from the commandline, all of that help documentation is also available
+online on our [wiki](https://github.com/microsoft/PowerShellForGitHub/wiki).
+
+----------
+
 ## Logging
 
 All commands will log to the console, as well as to a log file, by default.