feat(docs): add readme

Author: yoshuawuyts

README.md

@@ -0,0 +1,29 @@
+# postcss-prefix
+Replace `:host` elements with a prefix of your choosing
+
+## Usage
+```js
+const prefix = require('postcss-prefix')
+const postcss = require('postcss')
+
+const css = `
+  :host { color: blue }
+  .hello { color: black }
+`
+
+const newCss = postcss()
+  .use(prefix('#hello-world'))
+  .process(css)
+  .toString()
+
+console.log(newCss)
+// => #hello-world { color: blue }
+// => .hello { color: black }
+`
+```
+
+## See Also
+- [sheetify](https://github.com/stackcss/sheetify)
+
+## License
+[MIT](https://tldrlegal.com/license/mit-license)