Now that you've gotten over the excitement of WhatsApp scrapping its $1 annual fee
There are three ways, actually. Tap and hold on a blank area of the home screen, choose Widgets and then tap and hold on the WhatsApp Chat option, dragging it to a spare spot on screen-the app then prompts you to pick a conversation. Alternatively, press and hold on a conversation in WhatsApp and choose Add chat shortcut from the list (a widget icon is added on the first available home screen spot), or choose Add shortcut from the context menu inside a conversation.
It's useful for quickly jumping to conversations with your favorite people or groups you've set up for work, family, and so on, without having to navigate through the WhatsApp front screen first (particularly if the conversations are old ones). The other WhatsApp widgets for Android let you jump quickly to the camera component or show recent messages on the home screen.