[doc] Update README with reference to Flight Recorder

[c-p-i-o]

Summary:
Update readme with reference to the flight recorder tutorial to help users diagnose stuck jobs.

Test Plan:
none.

[lessw2020]

looks good!
I have feedback on the tutorial itself (not the readme add).
You break up the flow of the document here:

An example command is:

Caveat: tabulate module is needed, so you might need pip install it first.

Two suggestions:
a - don't interrupt the flow - move from 'an example command is:' to the actual command. Don't distract in the middle...i.e. Move the caveat to afterwards.

b - you tell the user they might need to pip install tabulate module...so please show the exact command. Is it 'pip install tabulate' or is it 'pip install tabulate module' or is it 'pip install tabulate-module' or ?

[c-p-i-o]

looks good! I have feedback on the tutorial itself (not the readme add). You break up the flow of the document here:

An example command is:

Caveat: tabulate module is needed, so you might need pip install it first.

Two suggestions: a - don't interrupt the flow - move from 'an example command is:' to the actual command. Don't distract in the middle...i.e. Move the caveat to afterwards.

b - you tell the user they might need to pip install tabulate module...so please show the exact command. Is it 'pip install tabulate' or is it 'pip install tabulate module' or is it 'pip install tabulate-module' or ?

Thanks! I've create pytorch/tutorials#3081 with these suggestions!