Description of SimplePie v1.5.6 library import into Moodle

Obtained from https://github.com/simplepie/simplepie/releases/

To upgrade this library:
1. Download the latest release of SimplePie from https://github.com/simplepie/simplepie/releases/
2. Remove everything inside lib/simplepie/ directory except readme_moodle.txt (this file) and moodle_simplepie.php.
3. Extract the contents of the release archive into a directory.
4. Move the following files/directories from the extracted directory into lib/simplepie:
    - library/
    - autoloader.php
    - LICENSE.txt
    - README.markdown
5. That should leave you with just the following. Do not move them. If there is any difference, check if they also need to be moved and update this doc:
    - idn (This is a third-party library that SimplePie can optionally use. We don't use this in Moodle)
    - CHANGELOG.md
    - composer.json
    - db.sql

Changes:
  * None. This import contains _NO_CHANGES_ to the simplepie.inc file, changes are
    controlled through OO extension of the classes instead.
