New tutorials: write section to describe sources of packages

(For more information about this issue, please see #194).

We need a short section to follow the "What's a package?" section (see #199) (i.e., as its own file). It should satisfy the following goals (in no particular order):

• explain that packages can be distributed through many channels (e.g., downloaded from PyPI with a tool like pip, an archive downloaded from a website, a VCS repository)
• explain that PyPI is an extremely popular way of getting packages but not the only way
• explain that PyPI is an index of packages, but not the only possible index
• illustrate the point with an example (maybe mention a popular package, then describe how it can be found on PyPI, in a VCS repository, and downloaded from the project's website)

In short, this section should answer the question, Where do packages come from?

When opening pull requests for this issue, please submit to the develop branch. If you have questions that concern contributing more generally, please use issue #194. Otherwise, feel free to comment with questions or feedback. Thanks!

Start by reading the existing "What's a package?" section and the related issues #194 and #199. Add a short section as its own file explaining package distribution channels, PyPI and other indexes, and one package example covering PyPI, a VCS repository, and the project website. Submit the pull request to the develop branch.