Update contributing.md

Author: f3dm76

CONTRIBUTING.md

@@ -2,6 +2,16 @@
 
 ## Code Style
 
+Please use selected code formatting style:
+- 4 spaces tab
+- no spaces on empty lines
+- comment: "// start with small letter"
+- declaration: "var users: [User]"
+
+### swift-format
+
+You can use [swift-format](https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwjol7H-_6iLAxW5ZmwGHViJIX8QFnoECBoQAQ&url=https%3A%2F%2Fgithub.com%2Fswiftlang%2Fswift-format&usg=AOvVaw0kMi_vMj0IW_Vm5BZ8ffcT&opi=89978449) to do this
+
 Code style is specified in .swift-format
 Run swift-format before checkin to ensure matching code style