Add documentation for adding extensions

[colinrobinsonuib]

In my deploy/ folder I created a folder called extensions/ where I put some extensions (e.g. WikibaseQualityConstraints).

I then created docker-compose.override.yml that looks like:

services:
  wikibase:
    volumes:
      - ./extensions:/extensions:ro
    entrypoint: >
      sh -c "cp -r /extensions/* /var/www/html/extensions/ && /bin/bash /entrypoint.sh"

I feel like this would be something nice to put in the README for new people.

[welcome]

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[lucamauri]

Regarding extensions, I think that also the following configuration:

# Load extensions if present, alphabetically ordered by filename
foreach (glob("LocalSettings.d/*.php") as $filename)
{
        include $filename;
}

needs a bit of explanation in the documentation.