doc(readme): add some content to readme

README.md

-# react-admin-cubicweb
+# React-Admin-CubicWeb
+
+React-Admin-CubicWeb (`ra-cubicweb`) is a proof of concept to use
+react-admin with a CubicWeb instance. It aims to provide:
+
+* an implementation of a data provider allowing to use data from CubicWeb
+  with React-admin;
+* default admin views in order to manage CubicWeb entities (displaying,
+  creation, edition).
+
+If this proof of concept is conclusive, React-Admin-CubicWeb could replace
+the legacy views of CubicWeb for administration interface.
+
+React-admin (https://marmelab.com/react-admin/) is a React Web framework
+(with typescript bindings) allowing to simply build administration interfaces.
+
+## How does it work ?
+
+React-admin relies on Resource components. A Resource component is a React
+component allowing to declare views in order to display, create and edit
+instances of concept.
+
+For each resource component, it creates routes to adequate pages. To load data
+it uses a data provider which implements data fetching from a given source of
+data.
+
+In our case, we aims to define a data provider allowing to fetch data from any
+CubicWeb instance.
+
+## Usage overview
+
+We need:
+
+* a CubicWeb instance with cubicweb-rqlcontroller
+  (https://forge.extranet.logilab.fr/cubicweb/cubes/rqlcontroller/);
+* a Javascript object which represents the yams schema of the aforementioned
+  instance;
+* to create a React application which will use ra-cubicweb resources
+  and data provider.