-add - If two create_clock assignments are applied to the same target, the second assignment will be ignored and a warning will be issued. This option on the second assignment
means that it describes a second clock coming into the device. An example where this is used is
if a device plugs into two different boards, and the legacy board might drive a slower clock into
the FPGA. This allows TimeQuest to analyze both scenarios.
add意思简单说就是再让timequest分析一条时钟。当你fpga换主时钟代码没变,可以用这个