Add more android interview questions and answers

Author: RotBolt

README.md

@@ -83,6 +83,9 @@
 * **What is retained `Fragment`?**
     - By default, Fragments are destroyed and recreated along with their parent Activity’s when a configuration change occurs. Calling setRetainInstance(true) allows us to bypass this destroy-and-recreate cycle, signaling the system to retain the current instance of the fragment when the activity is recreated.
 
+* **What is the purpose of `addToBackStack()` while commiting fragment transaction?**
+    - By calling addToBackStack(), the replace transaction is saved to the back stack so the user can reverse the transaction and bring back the previous fragment by pressing the Back button. For more [Learn from here](https://stackoverflow.com/questions/22984950/what-is-the-meaning-of-addtobackstack-with-null-parameter)
+
 #### Views and ViewGroups
 
 * **What is `View` in Android?** - [Learn from here](https://blog.mindorks.com/android-user-interface-view-components)
@@ -440,6 +443,8 @@
 
 * **Design File Downloader** - [Learn from here](https://github.com/MindorksOpenSource/PRDownloader)
 
+* **Design an Image Loading Library** - [Learn from here](https://medium.com/@maheswaranapk/android-create-your-own-image-loading-library-in-kotlin-diy-bc7be9f286c5)
+
 * **HTTP Request vs HTTP Long-Polling vs WebSockets** - [Learn from here](https://www.youtube.com/watch?v=k56H0DHqu5Y)
 
 ### Android Unit Testing