Mention autoloading on Addons page

[duncanmcclean]

Autoloading was recently added for addons, meaning things like fieldtypes, routes, etc don't need to be manually registered, as long as the addon follows convention.

We should probably mention autoloading on the Addons page, but caveat it that your users will need to running Statamic 5.28.0 or above, otherwise you should still be registering stuff manually.

Related: statamic/cms#9270