docs: add some documentation

Author: fippli

README.md

@@ -18,11 +18,23 @@ const Foo = ({ name, commonProp1 }) => (
 );
 
 <ComponentMap
+  // Component that should be mapped
   component={Foo}
+
+  // Array with properties for each
+  // mapped child
   data={[{ name: "hello" }, { name: "world" }]}
+
+  // Common properties shared among all
+  // the children e.g. state, dispatch
+  // and callback functions like onClick
   passProperties={{
     commonProp1: "commonProp1",
   }}
+
+  // Each react child should have a unique
+  // key prop
+  keyFunction={(props, index) => props.name}
 />
 
 ```