README with description for Querying the DOM

Also includes link to README from Interacting with the User; exercise
titles
All exercises for Querying the DOM

Covers querySelector and querySelectorAll for finding elements by id,
class, tag name, and name attribute.

Also includes an exercise to learn the getElement(s)ByX methods.
Slight edit to last exercise: include link as reference
Edit exercise in Querying the DOM to practice indexing into NodeList
Fix semantic error s/id/class
Change search suggestions to use CSS keyword when appropriate
Link to Code Union wiki article on CSS selectors
Merge pull request #7 from codeunion/querying-the-dom

Querying the DOM