"Getting Started Guide - Chapter 2 Setting up LibreOffice - Choosing language settings - Install the required dictionaries" is rather useless. Heading "Install the required dictionaries" pretends to contain information concerning installation, but it only explains how to find dictionaries in the internet. If you have some existing pli.aff / pli.dic it does not tell you how to install.
I'll take care of this sometime next week
I've done the addition and hopefully it will be up soon. One note is that you can't install .aff or .dic file, you have to install from the oxt files that are on the site that you are directed to within LibreOffice. If you want to use a .dic or .aff file you'd have to convert it to an oxt. If this is needed/wanted in the manual, please create a new bug and direct me to it. I'm going to mark this one as complete, changes will be posted soon.
The addition has been accepted and will be included in the next manual (for 3.5). It's unlikely that it will be included in 3.3 or 3.4. Thanks for pointing this one out, hopefully the addition comes in useful to yourself and others :)