Add 'FocusBlur tab detection'

FocusBlur-tab-detection.md

@@ -0,0 +1,15 @@
+Detect tab change in react, class base:
+
+```
+componentDidMount() {
+    window.addEventListener("focus", this.onFocus)
+}
+
+componentWillUnmount() {
+    window.removeEventListener("focus", this.onFocus)
+}
+
+onFocus = () => {
+    //
+}
+```