feat: add plugin api wip

diff --git a/docs/core-concepts/plugins.md b/docs/core-concepts/plugins.md
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+# Plugins
+
+Pinia stores can be fully extended thanks to a low level API. Here is a list of things you can do:
+
+- Add new properties to stores
+- Add new methods to stores
+- Wrap existing methods
+- Change or even cancel actions
+- Implement side effects like local storage
+- Apply **only** to specific stores
+
+Plugins are added to pinia with `pinia.use()`:
+
+```js
+// add a property named `secret` to every store that is created after this plugin is installed
+pinia.use(() => ({ secret: '' }))
+```