Document environment variables for PyThaiNLP

Added environment variables section with descriptions and statuses.

Author: bact

README.md

@@ -72,7 +72,17 @@ To see the specific libraries included in each extra,
 please inspect the `[project.optional-dependencies]` section of
 [`pyproject.toml`](https://github.com/PyThaiNLP/pythainlp/blob/dev/pyproject.toml).
 
-## Data directory
+## Environment variables
+
+| Variable | Description | Status |
+|---|---|---|
+| `PYTHAINLP_DATA` | Path to the data directory (default: `~/pythainlp-data`). | Current |
+| `PYTHAINLP_DATA_DIR` | Legacy alias for `PYTHAINLP_DATA`. Emits a `DeprecationWarning`. Setting both raises `ValueError`. | Deprecated; use `PYTHAINLP_DATA` |
+| `PYTHAINLP_OFFLINE` | Set to `1` to disable automatic corpus downloads. Explicit `download()` calls still work. | Current |
+| `PYTHAINLP_READ_ONLY` | Set to `1` to enable read-only mode, which prevents implicit background writes to PyThaiNLP's internal data directory (corpus downloads, catalog updates, directory creation). Explicit user-initiated saves to user-specified paths are unaffected. | Current |
+| `PYTHAINLP_READ_MODE` | Legacy alias for `PYTHAINLP_READ_ONLY`. Emits a `DeprecationWarning`. Setting both raises `ValueError`. | Deprecated; use `PYTHAINLP_READ_ONLY` |
+
+### Data directory
 
 PyThaiNLP downloads data (see the data catalog `db.json` at
 [pythainlp-corpus](https://github.com/PyThaiNLP/pythainlp-corpus))
@@ -86,16 +96,6 @@ inside the function that will be distributed to worker nodes.
 See details in
 [the documentation](https://pythainlp.org/dev-docs/notes/installation.html).
 
-## Environment variables
-
-| Variable | Description | Status |
-|---|---|---|
-| `PYTHAINLP_DATA` | Path to the data directory (default: `~/pythainlp-data`). | Current |
-| `PYTHAINLP_DATA_DIR` | Legacy alias for `PYTHAINLP_DATA`. Emits a `DeprecationWarning`. Setting both raises `ValueError`. | Deprecated; use `PYTHAINLP_DATA` |
-| `PYTHAINLP_OFFLINE` | Set to `1` to disable automatic corpus downloads. Explicit `download()` calls still work. | Current |
-| `PYTHAINLP_READ_ONLY` | Set to `1` to enable read-only mode, which prevents implicit background writes to PyThaiNLP's internal data directory (corpus downloads, catalog updates, directory creation). Explicit user-initiated saves to user-specified paths are unaffected. | Current |
-| `PYTHAINLP_READ_MODE` | Legacy alias for `PYTHAINLP_READ_ONLY`. Emits a `DeprecationWarning`. Setting both raises `ValueError`. | Deprecated; use `PYTHAINLP_READ_ONLY` |
-
 ### Offline mode
 
 Set `PYTHAINLP_OFFLINE=1` to disable **automatic** corpus downloads.