clarified definition of Activity

One of the todos in the meeting minutes of the recent workgroup meeting.
Added an example to clarify possible confusion about interpretation of
'Activity'.
added descriptions for MUST SHOULD MAY

These three words are used throughout the document, but a description
was nowhere given.
As a side note: I'm not a native speaker of English so apart from
correctness and completeness, please check grammar.
added entry MUST SHOULD MAY to the index of definitions

however I suggest leaving the index of definitions out of the document
completely, since the list is not long at all and immediately below the
index, so it doesn't add very much.
Merge pull request #49 from paul-esch/master

Chapter 3: Definitions