Add new field to custom content entity type
After installation, you need to handle updating the schema yourself. There are some different scenarios, some are more easy to deal with than others. The most complex is moving data from one field type to another which means you would have to do a migration of data.

Anyways Drupal core has the entityDefinitionUpdateManager() which can be accessed doing \Drupal::entityDefinitionUpdateManager(). This manager can do a lot of the things you need to do when you change the base fields of content entities.

Creation of a new field is pretty easy, and all you need to make your own update hook:

function hook_update_8xxx() {
  \Drupal::entityDefinitionUpdateManager()-&gt;applyUpdates();
}


This will apply all updates to all entities. This can generally be used for most things, but if there are data in Drupal has a failsafe that throws an exception to make sure it doesn't accidentally mess data up. During the beta the system attempted to handle data migration as well, but it didn't work well so it was decided that this is something module maintainers should do themselves.