add info on low power based on the idle thread

[foldl]

It's better to give some information on low power based on the idle thread. An example (template) of the idle thread is given.

There are other RTOSes that have an idle task as default. For developers who are porting projects, with this information available, I think issues like #172 can be avoided.

[microsoft-cla-retired[bot]]

All CLA requirements met.

[goldscott]

Hi @foldl - we have updated the low_power.md documentation and it will be available in the next release, scheduled for the end of July. We do make the suggestion that an idle thread is one possible implementation, but we do not have an example of an idle thread.

[goldscott]

We updated the documentation in the 6.1.12 release: https://github.com/azure-rtos/threadx/blob/master/utility/low_power/low_power.md