doc: add stop to support .unitypackage section on README.md

- This library depends on System.Runtime.CompilerServices.Unsafe. But with `package manager` and `.unitypackage` it is hard to maintain both. so I deprecated to support .unitypackage.

Author: netpyoung

README.md

@@ -15,10 +15,6 @@
 
 choose your preference
 
-### using .unitypackage
-
-- [Download this .unitypackage from Release Page](https://github.com/netpyoung/unity.webp/releases)
-
 ### using OpenUPM
 
 As a shared dependency, this uses the openUPM package `com.system-community.systemruntimecompilerservicesunsafe`, you will need to add it to OpenUPM's scopes, and it will be pulled in by this package.
@@ -67,6 +63,11 @@ ex)
   - ref: <https://forum.unity.com/threads/async-await-and-webgl-builds.472994/>
 - For WebGL build, this repo copied [webmproject/libwebp](https://github.com/webmproject/libwebp) directly. It will be more cleanable if it can be support prebuilt. But It needs more investigate.
 
+
+## stop to support .unitypackage
+
+- This library depends on System.Runtime.CompilerServices.Unsafe. But with `package manager` and `.unitypackage` it is hard to maintain both. so I deprecated to support .unitypackage.
+
 ## LICENCE
 
 ### webp