Add documentation

Author: Jos Koelewijn

RuleDocumentation/AvoidUnInitializedVarsInNewRunspaces.md

@@ -0,0 +1,27 @@
+# AvoidUnInitializedVarsInNewRunspaces
+
+**Severity Level: Warning**
+
+## Description
+
+If a scriptblock is intended to be run as a new runspace, variables inside it should use $using: directive, or be initialized within the scriptblock.
+
+## How to Fix
+
+Within `Foreach-Object -Parallel {}`, instead of just using a variable from the parent scope, you have to use the `using:` directive:
+
+## Example
+
+### Wrong
+
+``````PowerShell
+$var = "foo"
+1..2 | ForEach-Object -Parallel { $var }
+``````
+
+### Correct
+
+``````PowerShell
+$var = "foo"
+1..2 | ForEach-Object -Parallel { $using:var }
+``````