Make README more accessible to novices (rspec#2042)

* Make README more accessible to novices

* Amend per @JonRowe's review

* Apply second round of fixes per @JonRowe's review

* Amend per @benoittgt's code review

* Remove trailing whitespace

* Apply more fixes per @JonRowe's review

* Change semicolon to full colon in intro

* Fix punctuation in intro

* Apply changes per @samphippen's review

* Fix description of spec type assignment, per @JonRowe's suggestion

* Rephrase introduction per @mikegee's review

Author: rlue