Add information to Calendar page

[sseymore]

From UX testing:

3. Find all of the newspaper titles that were issued on February 7, 1930: Most users found the Calendar tab and used that. Some useful feedback included lack of contextual information on the Calendar page and also not realizing that selecting a year in the drop down does not automatically update the calendar and results. Clicking the Go button is necessary to update the search results.
   Recommendation: Add a sentence or two about the functionality and purpose of the Calendar page and also remind users to click Go up update results for the chosen year.

https://oregonnews.uoregon.edu/issues/

Add text to the top of this page:

Browse by date by selecting the year in the drop-down menu. Click Go to update the calendar dates below.