README.md doesn't describe optional tests

[karenetheridge]

If you look inside the draft directory, there are a number of .json files,
which logically group a set of test cases together. Often the grouping is by
property under test, but not always, especially within optional test files
(discussed below).

...only there is no discussion below.

This paragraph was introduced in commit 9cda89e way back in 2012, but no description of the optional/ directory was added.

A simple paragraph should be enough -- something like "Tests in the optional/ directory should not be run automatically. They describe behaviour that may be implemented in accordance with section 5 of RFC2119 [that's the definition of "MAY"]" perhaps?

In my code that wraps around the test suite, I intend to run these tests only if include_optional = 1 was included as an option.

[Julian]

Thanks! I agree, but this is #25

A PR would of course be more than welcome!