Update grafana/ui readme

packages/grafana-ui/README.md

-# Grafana (WIP) shared component library
+# Grafana UI components library
 
-Used by internal & external plugins.
+@grafana/ui is a collection of components used by [Grafana](https://github.com/grafana/grafana)
+
+Our goal is to deliver Grafana's common UI elements for plugins developers and contributors.
+
+See [package source](https://github.com/grafana/grafana/tree/master/packages/grafana-ui) for more details.
+
+## Installation
+
+`yarn add @grafana/ui`
+
+`npm install @grafana/ui`