Explain background vs. import

[rebloor]

In a comment on issue 97 against the with extension examples a request was made about adding ES6/ES7 code transpilation examples, or explicitly about importing JavaScript.

The author of the comment has confirmed that his query was actually answered by a clarification over the behavior of the manifest key background.

Update documentation to provide clarification on MDN.

[rebloor]

Unfortunately, the StackOverflow post providing clarification on the use of the manifest key background has been deleted. Having reviewed the other related material, it's unclear what this clarification may have been.