Make Windows support more obvious

[stefnotch]

I read the flamegraph-rs readme, and couldn't quite tell if Windows is supported. This edits the Readme to make it more obvious.

Though, with this edit, it no longer pushes users to try out dtrace on Windows. Is that good or bad?

[djc]

This should probably mention both the dtrace and the blondie backends, and ideally make a recommendation which of the two is more useful/reliable.

[stefnotch]

Okay, I reworked the Readme a bit more, let me know what you think. I tried to imitate the readmes of other popular projects, like this one.

Regarding the backends, is blondie the more reliable one? That was my best guess after reading the issue tracker. If yes, then I'll add the recommendation.

[djc]

Sorry, it's a little hard to review these changes since it's a whole bunch of stuff mashed together. Would prefer small commits that make one logical change at a time, which makes it easier to review, so I can verify the changes against the motivation.

[stefnotch]

Okay, I'll rewrite the git history and let you know once I'm done. I was hoping that it's short enough to review in one go, but I suppose having a more granular changeset is helpful.

[djc]

Thanks -- sorry, I don't have a ton of time to devote to flamegraph so trying to make this efficient.