Update readme.md

- added lambda intro

Author: tboychuk

lambdas/readme.md

@@ -8,3 +8,25 @@ You're supposed to be familiar with OOP, have basic knowledge of JDK, and be abl
 ### See also :point_down:
 * [Tutorial on Optional](https://github.com/bobocode-projects/java-8-tutorial/tree/master/optional)
 * [Tutorial on Stream API](https://github.com/bobocode-projects/java-8-tutorial/tree/master/stream-api)
+##
+Java is an OOP language, so it always works with classes and **doesn't support stanalone functions**. In case you want to **pass some function as a method parameter**, or **store some code into a variable**, you should use a *Funatinal Interface* and a *Lambda expression*. 
+
+* A *Functional Interface* represents a **function signature**. It contains only one abstract method.
+* A *Lambda expressoin* respresents a **function body**. Is an anonymous function that implements the abstract method of the functiona interface
+
+The purpose of the lambda and functional interfaces is to **make it easier to create function objects** and provide an **ability to use some functional programming technics in Java.**
+
+A typical example is interface `Comparator<T>`:
+
+```
+        accounts.sort(new Comparator<Account>() {
+            @Override
+            public int compare(Account o1, Account o2) {
+                return o1.getFirstName().compareTo(o2.getFirstName());
+            }
+        });
+```
+It can be easily simplified using lambda expression:
+```
+        accounts.sort((a1, a2) -> a1.getFirstName().compareTo(a2.getFirstName()));
+```