Change README.md reference of Graceful Cleanup

[kostaslamo]

Operating System

• Linux
• Windows 7
• Windows 10
• [ x] MacOS
• other:

NodeJS Version

• 0.x
• 4.x
• 6.x
• 7.x
• other: 12

Tmp Version

TBD:Enter tmp version here
latest

Expected Behavior

TBD:What have you expected tmp to do?
Delete "temporary" files and folders on process exit

Experienced Behavior

TBD:What did actually happen?
Did not delete "temporary" files on process exit.(I haven't read in the end of the Readme.md that i should
use tmp.setGracefulCleanup() as you mention that files and directories are removed on process exit in the first lines and i was expecting this behaviour)

Please change the position of Graceful cleanup on README.md maybe on the beginning so that it will be clear that this should be used in order that tmp files and dirs are cleaned up in process exit.