Add spec for prop logic
Fix spelling and grammar in README

As well as fixing spelling and grammar, I made these changes:

* reworded two parts slightly – changing “one” to “language”, and merging “you can try it” into the previous paragrah – because I think they are easier to read that way
• changed the formatting of the questions from heading level 2 to level 3, because with GitHub’s formatting, level 2 has a horizontal line under it, and those lines made the document seem less organized
• made “the `develop` branch” a link to viewing that branch on GitHub
Merge pull request #58 from roryokane/patch-1

Fix spelling and grammar in README
Merge pull request #67 from gogotanaka/develop

Update README.md and add specs