include scrape source for rosters in documentation

[matthewdwood82]

1. Is your feature request related to a problem? Please describe.
   The source of the data which populates the individuals on the 53-man roster for a given week is not clear.

2. Describe the solution you'd like
   Add 1-2 lines in load_rosters() or a dependency that indicates the source for this data.

3. Describe alternatives you've considered
   A field in the dataframe that explicitly references the datasource that was used to determine the player was on the 53-man for this week and year.

4. Additional context
   Had a friend ask me about source in a sports analytics chat I belong to, I did not have a satisfying answer for him.

[tanho63]

Hmm. It's a bit of a mix of things so not a straightforward question to answer. Is the hope to replicate it or to cite it?

[matthewdwood82]

Desire is to understand veracity of roster data in the package at kickoff for relevant week and year, especially for the past 1-2 year's data. Goal is to use it for an anti-points league, to validate whether a rostered player could have accumulated fantasy points.