Add documentation on interpreting tests (#508)

The project's CONTRIBUTING.md was written with test authors in mind. It
contains details on non-technical metadata (e.g. "author" and "es6id"),
helper function usage, and preferred code structure. In addition, it
elides certain low-level technical details on the requirements for the
runtime environment.

Introduce a new document targeted towards those executing the tests.
Formalize all expectations regarding how the runtime environment should
be defined, how metadata should be interpreted, and how results should
be understood. This information has overlap with the CONTRIBUTING.md
file, but it also contains details that are irrelevant to test authors.

This document can serve as a more formal contract between Test262 and
the implementors who consume it. This allows Test262 to unambiguously
document future modifications to the formal requirements which in turn
supports consumers who maintain their own test harnesses.