Tentative installer docs

Opencpn has a large set of plugins available. Most users need to have one or more plugins to get a usable installation. The installer is an effort to provide a simple way for end-users to install plugins. As such, it also makes it easier to package Opencpn for different platforms since the plugins doesn’t need to be packaged.

The installer has a basic contract with plugins as described in Tarballs and Catalog.

Here we have: