Added documentation for AvoidUsingWriteHost

RuleDocumentation/AvoidUsingWriteHost.md

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+#AvoidUsingWriteHost 
+**Severity Level: Warning**
+
+
+##Description
+
+It is generally accepted that you should never use Write-Host to create any script output whatsoever, unless your script (or function, or whatever) uses the Show verb (as in, Show-Performance). That verb explicitly means “show on the screen, with no other possibilities.” Like Show-Command.
+
+##How to Fix
+
+PTo fix a violation of this rule, please replace Write-Host with Write-Output.
+
+##Example
+
+Wrong： 
+
+```
+function Test
+{
+	...
+	Write-Host "Executing.."
+}
+```
+
+Correct: 
+
+```
+function Test
+{
+	...
+	Write-Output "Executing.."
+}
+
+function Show-Something
+{
+    Write-Host "show something on screen";
+}
+```