Added OpenUPM instructions

Author: yasirkula

.github/README.md

@@ -14,12 +14,14 @@ This plugin helps you place prefabs on a customizable grid easily.
 
 ## INSTALLATION
 
-There are 3 ways to install this plugin:
+There are 4 ways to install this plugin:
 
 - import [GridFramework.unitypackage](https://github.com/yasirkula/UnityGridFramework/releases) via *Assets-Import Package*
 - clone/[download](https://github.com/yasirkula/UnityGridFramework/archive/master.zip) this repository and move the *Plugins* folder to your Unity project's *Assets* folder
 - *(via Package Manager)* add the following line to *Packages/manifest.json*:
   - `"com.yasirkula.gridframework": "https://github.com/yasirkula/UnityGridFramework.git",`
+- *(via [OpenUPM](https://openupm.com))* after installing [openupm-cli](https://github.com/openupm/openupm-cli), run the following command:
+  - `openupm add com.yasirkula.gridframework`
 
 ## HOW TO