Add how to include pre-installed themes/plugins for WordPress

[wglambert]

Relevant to docker-library/wordpress#344 and docker-library/wordpress#341

As mentioned previously in docker-library/wordpress#246 (comment) and docker-library/wordpress#146; mounting the themes/plugins to /usr/src/wordpress/wp-content/ was advised, however there are issues notably with themes that cause a Stylesheet is missing. even when insuring the directory and it's contents are owned by www-data and given 777 permissions. It seems to be related to the tar cf - --one-file-system -C /usr/src/wordpress . | tar xf - in the entrypoint

/var/www/html/wp-content/ doesn't have this problem, it only requires read/write/execute for either the user www-data or for "other"

[tianon]

Looks great, although the formatting check is likely to fail in a moment (I think you need a blank line before the list items, at a minimum). You should be able to run ./markdownfmt.sh -w wordpress/content.md on your local copy if you want the script to simply correct the formatting for you, although it does sometimes get it wrong.