web-app implementation: the root reducer and the selectors

Rewritten-docs.md

@@ -3486,6 +3486,15 @@ and it creates a new state. This process is very easy to reason about and is
 also very easy to test using unit tests. Please read the [redux
 documentation](http://redux.js.org/) for detailed information about the library.
 
+The main difference between *Flux* and *redux* is the fact that there is only
+one store with one reducer in redux. The single reducer might be composed from
+several silmple reducers which might be composed from other simple reducers as
+well, therefore the single reducer of the store is often refered to as the root
+reducer. Each of the simple reducers receives all the dispatched actions and it
+decide which actions it will process and which it will ignore based on the
+*type* of the action. The simple reducers can change only a specific subtree of
+ the whole state tree and these subtrees do not overlap.
+
 ![Redux state handling schema](https://github.com/ReCodEx/wiki/raw/master/images/redux.png)
 
 ##### Redux Middleware
@@ -3513,7 +3522,19 @@ We created two other custom middleware functions for our needs:
 
 ##### Accessing The Store Using Selectors
 
-@todo
+The components of the application are connected to the redux store using a
+higher order function `connect` from the *react-redux* binding library. This 
+connection ensures that the react components will re-render every time some of 
+the specified subtrees of the main state changes.
+
+The specific subtrees of interest are defined for every connection. These
+definitions called *selectors* and they are are simple pure functions which take
+the state and return its subtree. To avoid unnecesary re-renders and selections 
+a small library called [reselect](https://github.com/reactjs/reselect) is used. 
+This library allows us to compose the selectors in a similar way the reducers
+are composed and therefore simply reflect the structure of the whole state
+tree. The selectors for each reducer are stored in a separate file in the
+`src/redux/selectors` directory.
 
 #### Routing