Custom code completion

Version 1.0 wrongly uses the directory that's used in the iOS apps:  ~/Library/Containers/com.textasticapp.textastic-mac/Data/Documents/#Textastic/CodeCompletion

I submitted version 1.1 yesterday which is going to use the directory you mentioned.

Is there some kind of tutorial for this? What could I add to this directory?

You can find all existing code completion definitions (JSON files) that are included in Textastic at https://github.com/blach/Textastic-Customization/tree/master/CodeCompletion

If you want to modify code completion definitions, you can edit them and put them in the "~Library/Containers/com.textasticapp.textastic-mac/Data/Library/Application Support/Textastic/CodeCompletion" folder mentioned above. Textastic will pick up the changes at application startup time, so you have to manually quit the app. 

If you do not change the uuid, your custom file will be used instead of the built-in one with the same uuid. If you use a new uuid (you can generate one with the uuidgen command line tool), the code completion file will be loaded in addition to existing ones.