Fix manifest

- Added non manifest files in packages to ignore
- Fixed package manifest

.gitignore

 MaterialGraphProject/Assets/Plugins*
 .DS_Store
 MaterialGraphProject/Assets/_MingWai/New Custom Texture.asset.meta
+MaterialGraphProject/Packages/*
+!MaterialGraphProject/Packages/manifest.json

MaterialGraphProject/Packages/manifest.json

 {
     "registry": "https://staging-packages.unity.com",
     "dependencies": {
-    	"com.unity.postprocessing": "0.4.0"
+    	"com.unity.postprocessing": "0.1.0"
     }
 }