Add doc folder

doc/TODO.txt

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+TODO: auto-timestamps
+- SaveXyz should set created_at and updated_at; shouldn't touch is_deleted or deleted_at
+- if soft delete is enabled, DeleteXyz should do update (not delete) and set is_deleted and deleted_at
+- SaveXyz shouldn't set created_at in the do-update branch
+- DeleteXyz should have pointer receiver
+- GetXyzByID should include `ErrItemIsDeleted` if item is soft-deleted
+
+TODO: primary-key
+- SaveXyz shouldn't set primary key if it's rowid
+
+TODO: foreign-key
+- Generated types should match foreign keys to the type of the column they point to
+
+TODO: modified-timestamps
+- set updated_at and created_at in SaveXYZ
+- soft delete option