add plugins list to docs

docs/plugins/Readme.md

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+
+The best thing about Slate is its plugin system, which lets you augment your editor however you want! Here's a list of known plugins for Slate. If you write your own plugin, pull request it in here!
+
+- [Plugins](#plugins)
+- [Serializers](#serializers)
+- [Utils](#utils)
+
+
+## Plugins
+
+- [`auto-replace-text`](https://github.com/ianstormtaylor/slate-auto-replace-text) ![](https://img.shields.io/npm/dm/slate-auto-replace-text.svg) — automatically replace a string of text with another when typed.
+
+
+## Serializers
+
+- [`html`](https://github.com/ianstormtaylor/slate-html) ![](https://img.shields.io/npm/dm/slate-auto-replace-text.svg) — a customizable HTML serializer.
+- [`paste`](https://github.com/ianstormtaylor/slate-paste) ![](https://img.shields.io/npm/dm/slate-auto-replace-text.svg) — an HTML serializer for pasting content from other websites with high accuracy.
+
+
+## Utils
+
+_There are no public utils yet, add one you've made!_