Update README.md

Adding steps for how to add the packages by name pulled from the manual here: https://docs.unity3d.com/Packages/com.unity.sentis@1.5/manual/install.html

README.md

@@ -15,6 +15,11 @@ The model has 1.3 billion parameters.
 ## How to Use
 * Create a new scene in Unity 2023
 * Install `com.unity.sentis` version `1.5.0-pre.2` and `com.unity.nuget.newtonsoft-json` packages
+  * Open the Package Manager, navigate to Window > Package Manager.
+  * Click + and select Add package by name....
+  * Enter com.unity.sentis, and enter the version in the line just below that
+  * Click Add to add the package to your project.
+  * Do the same for com.unity.nuget.newtonsoft-json
 * Add the RunPhi15.cs file to the Main Camera
 * Put `phi15.sentis`, `vocab.json` and `merges.txt` in the Assets/StreamingAssets folder
 * Adjust some of the variables such as the `outputText` string to set the prompt