Add module level docs to flamegraph

[jonhoo]

There's a lot of useful info at the top of flamegraph.pl that we should incorporate into the (currently non-existant) docs for the flamegraph module. FlameGraph also has a history that it'd be worthwhile to refer to.

[AnderEnder]

@jonhoo,
Maybe it is better to incorporate this information into cli flamegraph help, e.g. AnderEnder@c1533a5

[jonhoo]

I think I'd probably want the information in both. And the subset to show in each place is probably slightly different. For example, history probably only makes sense on the library, and the same with description of the folded stack format.