Add docs on changes

README.md

@@ -1,5 +1,7 @@
 # yarn-plugin-nixify
 
+**Upgrading the plugin? See [UPGRADING.md](./UPGRADING.md)**
+
 Generates a [Nix] expression to build a [Yarn] v2 project (not using
 zero-install).
 
@@ -129,6 +131,8 @@ Some additional settings are available in `.yarnrc.yml`:
   Preloading does mean another copy of dependencies on disk, even if you don't
   do local Nix builds, but the size is usually not an issue on modern disks.
 
+- `isolatedNixBuilds`, see [ISOLATED_BUILDS.md](./ISOLATED_BUILDS.md).
+
 [niv]: https://github.com/nmattia/niv
 
 ## Hacking