Added instruction for manual installation in ubuntu with instruction to create a desktop application

[UmangX]

1. added galaxy.png file as a temporary icon for desktop entry
2. added desktop file with for starting application at startup and dash and menu entry
3. added instructions on how to create a desktop file where to place it and where to place the icon file
4. added instruction on how to make the program executable and place in the path for starting it from the terminal