Added custom instructions for "Add People" modal

Added custom instructions for "Add People" modal.
Enter Unix names or Williams long-style email addresses. (Avoid short
email format: ob1@williams.edu)")
Examples: ob1, pleia, Anakin.Skywalker@williams.edu,
Jabba.T.Hutt@williams.edu
Merge pull request #23 from williamscollege/master

Added custom instructions for "Add People" modal