Add readme

Author: jonatansberg

README.md

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+# svelte-shortcodes
+
+Embed Svelte components inside rich content from any CMS using shortcodes. A
+concept we have shamelessly stolen [from WordPress](https://codex.wordpress.org/Shortcode_API).
+
+You can also use this module to replace matching html elements with a custom
+Svelte component of your choice.
+
+## Installation
+
+```sh
+yarn add --dev svelte-shortcodes # or npm i --save-dev svelte-shortcodes
+```
+
+## Usage
+
+The Shortcodes component will replace every occurrence of the component `name`
+with a Svelte component of your choice. Both WordPress-style shortcodes and the
+html tag names will be replaced.
+
+```
+<script>
+  import { Shortcodes } from "svelte-shortcodes";
+  import Heading from "./Heading.svelte";
+  import ResponsiveImage from "./ResponsiveImage.svelte";
+
+  let markup = `
+    [heading]Heading[/heading]
+    <img src="logo.png" alt="Svelte logo">
+    <p>Text...</p>
+  `;
+
+  // { [name: string]: Component }
+  let components = {
+    heading: Heading,
+    img: ResponsiveImage
+  };
+</script>
+
+<Shortcodes {components} {markup} />
+```
+
+Checkout the code inside `/dev` for a full example.
+
+## Development
+
+There is a dev app available inside the `/dev` folder that can be used for
+testing and experimenting with the components. To start it simply run:
+
+```sh
+yarn dev # or npm run dev
+```
+
+## Todo
+
+- [ ] Add tests
+- [ ] Consider separating html tag names from shortcodes
+- [ ] Find a better way to handle html fallback ([relevant svelte issue](https://github.com/sveltejs/svelte/issues/2324))