refactor: rename reactivity package name and APIs

packages/reactivity/README.md

@@ -0,0 +1,18 @@
+# @vue/observer
+
+## Usage Note
+
+This package is inlined into Global & Browser ESM builds of user-facing renderers (e.g. `@vue/runtime-dom`), but also published as a package that can be used standalone. The standalone build should not be used alongside a pre-bundled build of a user-facing renderer, as they will have different internal storage for reactivity connections. A user-facing renderer should re-export all APIs from this package.
+
+## Credits
+
+The implementation of this module is inspired by the following prior art in the JavaScript ecosystem:
+
+- [Meteor Tracker](https://docs.meteor.com/api/tracker.html)
+- [nx-js/observer-util](https://github.com/nx-js/observer-util)
+- [salesforce/observable-membrane](https://github.com/salesforce/observable-membrane)
+
+
+## Caveats
+
+- Built-in objects are not observed except for `Map`, `WeakMap`, `Set` and `WeakSet`.