Kotlin functions basic usage (#22231)

* Kotlin functions basic usage

* created index.md file

guide/english/kotlin/functions/index.md

@@ -0,0 +1,56 @@
+---
+title: Functions
+---
+
+# Functions
+
+### Basic Usage
+
+#### Declaration
+
+`fun` keyword is used to define the function  
+`a` and `b` are input parameters of type `Int`  
+`Int` (after brackets) is matching the return type  
+```kotlin
+fun sum(a: Int, b: Int): Int {
+   return a + b
+}
+```
+
+
+Same expression can be shortened (omit function brackets), meaning that return type is infered.  
+Return keyword can be also omited in such expression body
+```kotlin
+fun sum(a: Int, b: Int) = a + b
+```
+
+
+Return type can also be omitted (you don't have to use `void` return type) if function does not return anything
+```kotlin
+fun printSum(a: Int, b: Int) {
+    println("sum of $a and $b is ${a + b}")
+}
+```
+
+
+#### Usage
+
+
+Simple method call:
+```kotlin
+val result = sum(2, 3)
+```
+
+Named arguments in method call:
+```kotlin
+val result = sum(a = 1, b = 5)
+```
+
+Support of default arguments (if parameter is not provided in method call):
+```kotlin
+fun write(content: String, destination: String = "/tmp/") { ... }
+```
+
+#### Resources
+* [Basic Syntax Reference](https://kotlinlang.org/docs/reference/basic-syntax.html)
+* [Kotlin Functions Reference](https://kotlinlang.org/docs/reference/functions.html)