Update README.md

README.md

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 * (Temporary Extra Step) Download Terraria 1.3.5.2 from steam. [Instructions](https://github.com/tModLoader/tModLoader/issues/227#issuecomment-324450642)
 2. Open setup.bat in the root folder
     * If setup.bat won't open, you must unblock all the files in the cloned repository
+    * If you get an error that mentions File Cannot be Found, you might need to make sure `msbuild` is on your `PATH`. For a VS 2019 install, this should be in `C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Current\Bin`, so add that to your `PATH`. If you don't know how to edit your `PATH`, google it. Make sure to restart setup.bat after editing your PATH for it to take effect.
 3. Select your vanilla terraria.exe (must be vanilla) ([img](https://i.imgur.com/MccGyvB.png))
 4. Click on 'Setup' (top left button)
     * **Warning:** decompilation can take several hours to complete depending on your hardware. It's also likely that your computer **completely freezes** during the process, mainly once it hits NPC.cs It is recommended that you enable the 'Single Decompile Thread' option ([img](https://i.imgur.com/6mBbZnQ.png)) if you don't have very high end hardware. It's unwise to even attempt a decompile if you have less than 8 GB RAM. Having an SSD, powerful CPU and a high amount of RAM will significantly speed up the decompilation process.