Add data dictionary

DATA_DICTIONARY.md

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+# Data Dictionary
+
+Every field actually present in `US_Industrial_Facilities.csv` / `US_Industrial_Facilities.xlsx` (FACILITIES sheet). No field is included unless it is genuinely populated from EPA's own data.
+
+| Field | Type | May be empty | Description |
+|---|---|---|---|
+| `facility_name` | string | No | Name of the industrial facility as registered with EPA TRI. |
+| `operator` | string | No | Company/operator of record (parent company when reported, otherwise the facility's own registered name). |
+| `facility_type` | string | No | Program classification. All records in this dataset are TRI-regulated facilities (Toxics Release Inventory). |
+| `industry_activity` | string | No | General activity note: the facility manufactures, processes, or uses EPA-listed toxic chemicals above federal reporting thresholds. EPA's TRI facility endpoint does not expose a per-facility NAICS/SIC code, so none is fabricated here -- this field is a general program-level description, not a specific industry code. |
+| `state` | string | No | US state, 2-letter abbreviation. |
+| `city` | string | No | City where the facility is located. |
+| `address` | string | No | Street address of the facility. |
+| `source` | string | No | Always `EPA TRI (Toxics Release Inventory), data.epa.gov Envirofacts`. |
+| `source_id` | string | No | EPA's own TRI facility identifier for this record -- use this to cross-reference back to EPA's own public systems. |
+| `freshness` | string | No | Snapshot date and source-check note for this dataset (see README/QUICK_START for the exact date). |
+
+## Fields intentionally NOT included
+
+- **Any person-level contact information** (name, email, phone) -- this product is a facility/company intelligence dataset, not a contact or mailing list. No individual-level contact data is included in this edition, by design.
+- **Role/title of any contact** -- not applicable, since no contact data is included.
+- **Any industry classification code (NAICS/SIC)** -- not exposed by the specific EPA TRI endpoint this dataset is built from, so none is invented.
+- **Any purchase-intent, buying-signal, or "sales-ready" scoring** -- this dataset contains no such field. Any classification present is a factual, deterministic description of the facility's registered activity, never an inference about buying behavior.