Add a note that prefabs/assets require TMP.

Some of the prefabs and assets require text mesh pro, which isn't apparent as you start using things. We aren't able to automatically import TMP assets, so we should provide guidance that these are required.

Author: wiwei

Documentation/GettingStartedWithTheMRTK.md

@@ -47,6 +47,8 @@ Accept all the content and continue.
 
 > The MRTK also provides a [separate "Examples" asset](https://github.com/Microsoft/MixedRealityToolkit-Unity/releases), which currently contains demonstration scenes for current MRTK features.  In the future this will also contain educational "Example" scenes to aid with learning implementation tricks with the toolkit.
 
+Note that some prefabs and assets require TextMesh Pro, meaning you have to have the TextMesh Pro package installed and the assets in your project (Window -> TextMeshPro -> Import TMP Essential Resources).
+
 ### 3. Accept the Setup prompt (optional)
 ![](../External/ReadMeImages/MRTK_UnitySetupPrompt.png)