Simplified Package Manager installation instruction

Author: yasirkula

.github/README.md

@@ -21,8 +21,8 @@ There are 5 ways to install this plugin:
 - import [AssetUsageDetector.unitypackage](https://github.com/yasirkula/UnityAssetUsageDetector/releases) via *Assets-Import Package*
 - clone/[download](https://github.com/yasirkula/UnityAssetUsageDetector/archive/master.zip) this repository and move the *Plugins* folder to your Unity project's *Assets* folder
 - import it from [Asset Store](https://assetstore.unity.com/packages/tools/utilities/asset-usage-detector-112837)
-- *(via Package Manager)* add the following line to *Packages/manifest.json*:
-  - `"com.yasirkula.assetusagedetector": "https://github.com/yasirkula/UnityAssetUsageDetector.git",`
+- *(via Package Manager)* click the + button and install the package from the following git URL:
+  - `https://github.com/yasirkula/UnityAssetUsageDetector.git`
 - *(via [OpenUPM](https://openupm.com))* after installing [openupm-cli](https://github.com/openupm/openupm-cli), run the following command:
   - `openupm add com.yasirkula.assetusagedetector`