Add link to docs on decorators

[b-butler]

Issue Description

In the signac tutorial, we introduce the decorator syntax @ without defining what a Python decorator is or linking to documentation on decorators. A new user expressed some confusion at this point in the documentation. As decorators are a nice syntactic feature of Python, but potential not used commonly enough to be known by users who are newer to Python,

Proposed change

I think we should add a link to official docs on decorators (these are the best I could find https://www.python.org/dev/peps/pep-0318/ and https://docs.python.org/3/whatsnew/2.4.html?highlight=decorators#pep-318-decorators-for-functions-and-methods)

[vyasr]

I'm all for linking to good docs on decorators. I don't think we should attempt to explain them ourselves, but links are great.