Update Freezing pytest description in simple.rst

[pbrod]

I have trouble using third party plugins in my frozen program and discovered
that other people have experienced it as well:
https://mail.python.org/pipermail//pytest-dev/2015-June/003015.html

The problem is that the mechanism for plugin discovery used by pytest
(setupttools entry points) doesn't work with frozen executables so pytest
can't find any plugins. The solution seems to be to import the third party
plugins explicitly as shown here:
https://mail.python.org/pipermail//pytest-dev/2015-June/003018.html

This is not mentioned anywhere in the documentaion.

Thanks for submitting a PR, your contribution is really appreciated!

Here's a quick checklist that should be present in PRs:

• Add a new news fragment into the changelog folder
  • name it $issue_id.$type for example (588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the pr
  • ensure type is one of removal, feature, bugfix, vendor, doc or trivial
  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."
• Target: for bugfix, vendor, doc or trivial fixes, target master; for removals or features target features;
• Make sure to include reasonable tests for your change if necessary

Unless your change is a trivial or a documentation fix (e.g., a typo or reword of a small section) please:

• Add yourself to AUTHORS, in alphabetical order;

[nicoddemus]

Indeed, thanks for the PR! 👍

[coveralls]

Coverage remained the same at 92.558% when pulling 820ea6d on pbrod:patch-1 into cf9b31b on pytest-dev:master.