Document change of `MAX_URL_LENGTH` setting

[timobrembeck]

Changing the MAX_URL_LENGTH setting requires a database migration of the Url model.
Since this is not completely straight forward (or am I missing something?), I would add the recommended workflow for this to the README.

Similar as described in #137, I see two ways of dealing with this:

• monkeypatching their app label to be able to add linkcheck migrations in their own directory,
• or just running django-admin makemigrations linkcheck on the production system itself (and having to merge these individual migrations with the upstream ones in case we add more migrations)

Since both options don't seem ideal to me, I think we could also think about setting a sensible (?) but fixed value and don't allow this value to be configured?

[claudep]

I guess the basic issue is that the maximum possible length depends on the database backend, particularly with MySQL which have issues with longer lengths (https://docs.djangoproject.com/en/4.1/ref/databases/#character-fields). Same MySQL issue if we'd like to make it a TextField, due to unique=True.

As per the monkeypatching described above, there is https://docs.djangoproject.com/en/4.1/ref/settings/#std-setting-MIGRATION_MODULES.