Update README.md

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: lilfetz22

README.md

@@ -633,6 +633,14 @@ Edit `config.json` to change your field. You can use any field of expertise you
 - `"Machine Learning Operations"`
 - Any other field you specialize in
 
+**How field choice affects topic selection and cost**
+
+- The two pre-seeded fields  
+  - `"Data Science (Optimizations & Time-Series Analysis)"`  
+  - `"Generative AI & AI Agents"`  
+  use the `potential_topics` table in `database/topics.db` for topic selection. For these, the Topic Agent selects topics via SQLite queries only (no extra LLM call for topic selection).
+- When you use any other custom field (for example, `"Software Engineering (Cloud Architecture)"` or your own niche), the Topic Agent typically will not find matching topics in the database and will **fall back to LLM-based topic generation**. This requires an additional LLM API call and therefore increases cost for each run that needs a new topic.
+- If you want to use a custom field **without** paying the extra LLM cost for topic selection, you can seed your own topics into the database (for your custom field value) using `database/init_db.py` or a similar initialization step. Once seeded, your custom field behaves like the pre-seeded ones and uses database queries first.
 ### Character Limits
 
 The system enforces a 3000-character limit for LinkedIn posts. If a post exceeds this, it's automatically sent back to the Writer Agent for shortening.