Add an example of the pstats macro to README (closes #21)

[bicycle1885]

It also slightly shortens comments following each measurement, which improves visibility in narrow window and on printed paper.

[chriselrod]

Perhaps add an example showing specifying options? It is CPU dependent. The default settings always cause warnings for me, so I always do something like

@pstats "cpu-cycles,(instructions,branch-instructions,branch-misses),(task-clock,context-switches,cpu-migrations,page-faults),(L1-dcache-load-misses,L1-dcache-loads,L1-icache-load-misses),(cache-misses,cache-references),(dTLB-load-misses,dTLB-loads)" begin
    rand(mt, 1_000_000)
end

[bicycle1885]

Thank you, @chriselrod. I added an example to specify events to README.
I'm going to merge this because it's not a big deal anyway.