Require a result line after each test in README.

[kocielnik]

Doctest reported unindicative errors whenever a result line was missing after a test.

The cause was identified and a paragraph was added about it in the README for future reference.

[kocielnik]

Thanks for the awesome work keeping the JS DocTest alive and up-to-date!

[davidchambers]

Thank you for the pull request and for the words of encouragement, @kocielnik. :)