docs(README): Add a note that mixin.js is patched on Magento >= 2.3.6

Because:

- [the PR](magento/magento2#25587) for the fix has been merged and released in Magento 2.3.6 (?) 
- [this repo](https://github.com/integer-net/magento2-requirejs-bundling) also says no patches are needed since Magento 2.3.6

Author: jtomaszewski

README.md

@@ -20,7 +20,7 @@ _\* All data gathered on clean Magento 2 with sample data installed._
 Here are the requirements for Magepack to work:
 
 1. You need Node.js version 10 or higher installed.
-2. You need to have `mixins.js` module patched [(patch provided and explained here)](https://github.com/magento/baler/issues/23).
+2. If you are using Magento 2.3.5 or lower, you need to have `mixins.js` module patched [(patch provided and explained here)](https://github.com/magento/baler/issues/23).
 3. If you are using Magento 2.3.3 or lower, you need `jquery.cookie` module shim [(patch provided and explained here)](https://github.com/magento/baler/issues/6).
 4. [Magepack Magento module](https://github.com/magesuite/magepack-magento) installed.