Create README.MD

- added Stream API tutorial

Author: tboychuk

stream-api/README.MD

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+# <img src="https://raw.githubusercontent.com/bobocode-projects/resources/master/image/logo_transparent_background.png" height=50/> Stream API tutorial
+
+This is the tutorial on Stream API and functional programming techniques
+### Pre-conditions :heavy_exclamation_mark:
+You're supposed to be familiar with OOP, have basic knowledge of JDK, and be able to write Java code. 
+### Related exercises :muscle:
+*
+### See also :point_down:
+* [Tutorial on Lambdas](https://github.com/bobocode-projects/java-functional-features-tutorial/tree/master/lambdas)
+* [Tutorial on Optional](https://github.com/bobocode-projects/java-functional-features-tutorial/tree/master/optional)
+##
+*Stream API* provide an ability to **process sequences of data elements in a declarative way** and **simplify the task of 
+performing operations in parallel**
+
+The simplest example is the task of filtering a collection. Using **imperative** old-style approach we specify **HOW the task 
+should be done**. E.g. dealing with iteration, and storing each element in a new `ArrayList`. This way of processing is also 
+called *external iteration*. *Stream API* and it's **declarative** approach allows us to specify **WHAT should be done**, without 
+actually dealing with iteration and elements. This approach is also called *internal iteration*. 
+
+Imperative style:
+```java
+        List<Account> gmailAccounts = new ArrayList<>();
+        for (Account account : accounts) {
+            if (account.getEmail().endsWith("@gmail.com")) {
+                gmailAccounts.add(account);
+            }
+        }
+```
+Declarative style using *Stream API*:
+```java
+        List<Account> gmailAccounts = accounts.stream()
+                .filter(a -> a.getEmail().endsWith("@gmail"))
+                .collect(toList());
+```