Update README to provide more clear examples

[alissa-huskey]

I updated the README with some of my own code examples and did a little cleanup/polish.

• Replace "Quickstart" section with the code for the simplest possible use-case.
• Replace WSGI example, as I wasn't able to get the previous example working.
• Rename the previous "Quickstart" section to "How-To".
• Rename "Available funcargs" section to "Fixtures". (That seems to be the more common term. In any case, I didn't previously know that "funcargs" was another term for "fixtures", so it may be more clear to others as well.)
• Use code directives instead of literal blocks to syntax highlight python code.

[diazona]

Hi @alissa-huskey, thanks for sending a contribution! I'm sorry to keep you waiting; I haven't been active on GitHub for a little while (because real life got busy) and I didn't see the notification about your PR.

If you don't mind waiting a bit longer, I'd like to come back to this and review it over the weekend when I can give it my full attention.

[diazona]

Thanks again! These changes mostly look good, I just made a few suggestions.

Sorry again for the delayed response; I'll try to keep checking back so I don't miss further activity here.

[alissa-huskey]

No worries about the delay! Real life happens. I tend to be on github in fits and starts anyway, so I never expect a prompt response, if any.

I believe I addressed all of your feedback. Additionally I changed the monkeypatch link to point to the stable pytest.org docs URL.

[diazona]

Good call on that monkeypatch change.

Thanks again; I appreciate both the contribution and your patience!

[diazona]

I think the CI failures are because of #110, so I'll merge that and then get this finalized.