Update README.md with prerequisite of Targeting Pack (#12896)

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## PR Checklist
* [x] Closes #12733
* [x] CLA signed. If not, go over [here](https://cla.opensource.microsoft.com/microsoft/Terminal) and sign the CLA

README.md

@@ -299,6 +299,7 @@ If you would like to ask a question that you feel doesn't warrant an issue
   * Universal Windows Platform Development
   * **The following Individual Components**
     * C++ (v142) Universal Windows Platform Tools
+* You must install the [.NET Framework Targeting Pack](https://docs.microsoft.com/dotnet/framework/install/guide-for-developers#to-install-the-net-framework-developer-pack-or-targeting-pack) to build test projects
 
 ## Building the Code