Fix minor issues in documentation

[clint-white]

Here are a few cleanups to the documentation, mostly the formatting not the substance:

• Update link to the Python documentation to the latest 3.x version instead of 3.5.
• Add missing markdown inline code formatting and links to other documentation as appropriate.
• Fix typos, add missing periods to the ends of sentences.
• Regenerate REAMDME.md with cargo-readme.

Note that the documentation is inconsistent in how it refers to its primary type, sometimes using Counter and sometimes "counter". That's probably fine, the standard library often does the same thing (Vec and "vector", etc.). This PR does not attempt to standardize to one or the other way of refering to it. However, where the documentation used "Counter" with a captial C but no inline code formatting, we have added the markdown formatting. If you would like to decide on a standard way to refer to counters and make the use consistent throughout the documentation, I would be happy to ammend this PR with any necessary changes.

[coriolinus]

Please do clean up the merge conflict, but the changes look good.

[coriolinus]

I don't intend to impose a standard way of referring to the type within the documentation; to me, counter (lowercase) is a somewhat more abstract term which would cover alternate implementations, and Counter (uppercase) refers only to this implementation.

[clint-white]

Ok, that makes good sense on the use of counter as well as Counter.

I resolved the merge conflict.

[coriolinus]

Thanks again!