Add the instruction of install this package as an add-on directly in Orange3 Canvas

[jiqicn]

It's now possible to have the package installed directly as an Orange add-on. Instructions should be added for that.

These are the steps:

• Pre: make sure a keyword "Orange3 add-on" is in the keyword list of the PyPI release.
• 1. Open Orange3 canvas
• 2. From the top menus, navigate to "Options" -> "Add-ons"
• 3. In the pop-up window, click on the "Add more..." button
• 4. In the sub-pop-up window, input the package name "orangearg" and click the "Add" button.
• 5. You will be able to find the add-on at the very end of the list.