Add uninstallation instructions.

UNINSTALLING.md

+# Uninstallation Instructions
+
+Open a *PowerShell* console with elevated privileges ("Run as Administrator").
+Launch `InstallUtil` as follows:
+
+```
+& "C:\Windows\Microsoft.NET\Framework\v4.0.30319\InstallUtil.exe" /uninstall C:\Tools\AutoTx\AutoTx.exe
+```
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