Include env var documentation in package script

doc/Package.md

@@ -23,6 +23,12 @@ for a full description of this file.
   -test         : Testing mode, separate builds, allow dirty working copy
   -dryrun       : Don't perform any actual actions, just report on what you would do
   -help         : Print this help
+
+Environment Variables:
+  UE4INSTALL   : Use a specific UE4 install.
+               : Default is to find one based on project version, under UE4ROOT
+  UE4ROOT      : Parent folder of all binary UE4 installs (detects version).
+               : Default C:\Program Files\Epic Games
 ```
 
 ## What the Script Does

ue4-package.ps1

@@ -48,6 +48,13 @@ function Write-Usage {
     Write-Output "  -test         : Testing mode, separate builds, allow dirty working copy"
     Write-Output "  -dryrun       : Don't perform any actual actions, just report on what you would do"
     Write-Output "  -help         : Print this help"
+    Write-Output " "
+    Write-Output "Environment Variables:"
+    Write-Output "  UE4INSTALL   : Use a specific UE4 install."
+    Write-Output "               : Default is to find one based on project version, under UE4ROOT"
+    Write-Output "  UE4ROOT      : Parent folder of all binary UE4 installs (detects version). "
+    Write-Output "               : Default C:\Program Files\Epic Games"
+    Write-Output " "
 }
 
 if ($src.Length -eq 0) {