docs on mod dev environment migration

Author: serenibyss

README.md

@@ -32,7 +32,28 @@ And many more to come! And of course, all of these features are toggleable via a
 Coming soon!
 
 ## How to Install (Existing Project)
-Coming soon!
+- Download the latest master as a zip file from [here](https://github.com/GregTechCEu/Buildscripts/archive/refs/heads/master.zip)
+- Rename your `build.gradle` file to `build.gradle.old`
+- Copy the [`build.gradle`](https://github.com/GregTechCEu/Buildscripts/blob/master/build.gradle), [`dependencies.gradle`](https://github.com/GregTechCEu/Buildscripts/blob/master/dependencies.gradle), and [`repositories.gradle`](https://github.com/GregTechCEu/Buildscripts/blob/master/repositories.gradle) files from this zip to your project
+- Copy the [`settings.gradle`](https://github.com/GregTechCEu/Buildscripts/blob/master/settings.gradle) file from this zip to your project and replace your current one
+- Copy all the options from the [`gradle.properties`](https://github.com/GregTechCEu/Buildscripts/blob/master/gradle.properties) file to your `gradle.properties` (or copy entirely if you did not have one). You can leave your existing options if you know you need them, otherwise they can likely be removed
+- Configure `gradle.properties` for your mod
+- Move the necessary `dependencies` and/or `repositories` from `build.gradle.old` to the respective files (`dependencies.gradle`, `repositories.gradle`)
+    - NOTE that if you enable the `includeWellKnownRepositories` option in `gradle.properties`, this build script will automatically have the following Maven locations, meaning you don't need to add them yourself in `repositories.gradle`:
+      1. Curse Maven
+      2. Modrinth Maven
+      3. BlameJared Maven
+      4. CleanroomMC Maven
+    - NOTE that if you enable the `includeCommonDevEnvMods` option in `gradle.properties`, this build script will automatically have the following Mods, meaning you don't need to add them yourself in `dependencies.gradle`:
+      1. JEI
+      2. The One Probe
+- Delete the `build.gradle.old` file
+
+### Advanced
+- If your project was using Mixins, you may get a new mixin config file generated as `mixins.{modid}.json`, if yours was not named this way. Currently, you will have to move your Mixin config options to this newly generated file. If this behavior does not suit your needs, feel free to open an issue and start a discussion on different behavior
+- If your project was using environment variables for CI deployments, see the `gradle.properties` file's comments to view the environment variable names this script checks for
+- If you have any additional build script code that you need to be applied, create an `addon.gradle` file and put it there
+- If your project has no dependencies, you can safely delete the `repositories.gradle` and `dependencies.gradle` files, they are optional
 
 ## How to Use
 ### Files