Add explanations

README.md

@@ -50,11 +50,27 @@ question catalog the decks built here are based on.
 
 Python 3.11+, standard library only. No third-party dependencies.
 
+## Explanations
+
+The back of each card optionally carries a terse English explanation
+of *why* the right answer is right. Explanations are not part of the
+BNetzA catalog — they're authored separately (by humans or AI agents)
+into `explanations.json` at the repo root. The build is non-blocking
+on this: questions without an entry just show no explanation block.
+Entries with `confidence < 7` render a small "low confidence" badge
+so learners know the reasoning is provisional.
+
+`EXPLANATIONS.md` is the editorial contract: schema, sourcing
+guidance, confidence scale, and the workflows an AI agent should
+follow when asked to add or improve entries.
+
 ## More
 
 - `CLAUDE.md` — project orientation, pipeline overview.
 - `DESIGN.md` — source-discovery notes, JSON schema, per-stage
   design contracts.
+- `EXPLANATIONS.md` — schema + workflows for the explanations
+  database.
 
 ## License