Resolves #39

Before the change?

1. Documentation example code does not elaborate upon the valid types for selector_type (which are id, name, xpath, css, link, jspath, as can be determined by trying to pass in helloworld as a selector_type and getting back an error message clarifying the valid options during terraform apply).
2. Documentation example code does not mention the existence of any of the 6 "assertion" step types.

After the change?

1. Left all examples of selector_type as id as before, but added an inline comment at the end of the first example listing all 6 valid selector_type values (id, name, xpath, css, link, jspath).
2. Added 6 new "assertion" step type examples to cover Documentation example code does not mention the existence of any of the 6 "assertion" step types (assert_element_present, assert_element_not_present, assert_element_visible, assert_element_not_visible, assert_text_present, assert_text_not_present).

Pull request checklist

• [ N/A ] Acceptance Tests have been updated, run (make testacc), and pasted in this PR (for bug fixes / features)
• [ X ] Docs have been reviewed and added / updated if needed (for bug fixes / features)

Acceptance Test Output

(n/a; .md examples only changed)

Does this introduce a breaking change?

• Yes
• [ X ] No