binary-search: regenerate README

exercises/binary-search/README.md

 so locating an item (or determining its absence) takes logarithmic time.
 A binary search is a dichotomic divide and conquer search algorithm.
 
+This exercise introduces [generics](https://docs.oracle.com/javase/tutorial/java/generics/index.html).
+To make the tests pass you need to construct your class such that it accepts any type of input, e.g. `Integer` or `String`.
+
+Generics are useful because they allow you to write more general and reusable code.
+The Java [List](https://docs.oracle.com/javase/8/docs/api/java/util/List.html) and [Map](https://docs.oracle.com/javase/8/docs/api/java/util/Map.html) implementations are both examples of classes that use generics.
+By using them you can construct a `List` containing `Integers` or a list containing `Strings` or any other type.
+
+There are a few constraints on the types used in generics.
+One of them is that once you've constructed a `List` containing `Integers`, you can't put `Strings` into it.
+You have to specify which type you want to put into the class when you construct it, and that instance can then only be used with that type.
+
+For example you could construct a list of `Integers`:
+
+`List<Integer> someList = new LinkedList<>();`
+
+Now `someList` can only contain `Integers`. You could also do:
+
+`List<String> someOtherList = new LinkedList<>()`
+
+Now `someOtherList` can only contain `Strings`.
+
+Another constraint is that any type used with generics cannot be a [primitive type](https://docs.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html), such as `int` or `long`.
+However, every primitive type has a corresponding reference type, so instead of `int` you can use [`Integer`](https://docs.oracle.com/javase/8/docs/api/java/lang/Integer.html) and instead of `long` you can use [`Long`](https://docs.oracle.com/javase/8/docs/api/java/lang/Long.html).
+
+It can help to look at an [example use case of generics](https://docs.oracle.com/javase/tutorial/java/generics/types.html) to get you started.
+
+
 
 To run the tests: