docs: schema add --polars mode; expanded description

Author: jqnatividad

README.md

@@ -74,7 +74,7 @@
 | [reverse](/src/cmd/reverse.rs#L2)<br>📇🤯 | Reverse order of rows in a CSV. Unlike the `sort --reverse` command, it preserves the order of rows with the same key. If an index is present, it works with constant memory. Otherwise, it will load all the data into memory. |
 | <a name="safenames_deeplink"></a>[safenames](/src/cmd/safenames.rs#L2)<br>![CKAN](docs/images/ckan.png) | Modify headers of a CSV to only have ["safe" names](/src/cmd/safenames.rs#L5-L14) - guaranteed "database-ready"/"CKAN-ready" names.  |
 | [sample](/src/cmd/sample.rs#L2)<br>📇🌐🏎️ | Randomly draw rows (with optional seed) from a CSV using seven different sampling methods - [reservoir](https://en.wikipedia.org/wiki/Reservoir_sampling) (default), [indexed](https://en.wikipedia.org/wiki/Random_access), [bernoulli](https://en.wikipedia.org/wiki/Bernoulli_sampling), [systematic](https://en.wikipedia.org/wiki/Systematic_sampling), [stratified](https://en.wikipedia.org/wiki/Stratified_sampling), [weighted](https://doi.org/10.1016/j.ipl.2005.11.003) & [cluster sampling](https://en.wikipedia.org/wiki/Cluster_sampling). Supports sampling from CSVs on remote URLs. |
-| <a name="schema_deeplink"></a>[schema](/src/cmd/schema.rs#L2)<br>📇😣🏎️👆🪄 | Infer schema from CSV data, replete with data type & domain/range validation & output in [JSON Schema](https://json-schema.org/) format. Uses multithreading to go faster if an index is present. See [`validate`](#validate_deeplink) command to use the generated JSON Schema to validate if similar CSVs comply with the schema. |
+| <a name="schema_deeplink"></a>[schema](/src/cmd/schema.rs#L2)<br>📇😣🏎️👆🪄🐻‍❄️ | Infer either a [JSON Schema Validation Draft 2020-12](https://json-schema.org/draft/2020-12/json-schema-validation) or Polars schema from CSV data.<br>In JSON Schema Validation mode, it produces a `.schema.json` file replete with data type & domain/range validation. Uses multithreading to go faster if an index is present. See [`validate`](#validate_deeplink) command to use the generated JSON Schema to validate if similar CSVs comply with the schema.<br>With the `--polars` option, it produces a `.pschema.json` file that all polars commands (`sqlp`, `joinp` & `pivotp`) use to determine the data type of each column & to optimize performance.<br>Both schemas are editable and can be fine-tuned. For JSON Schema, to refine the inferred validation rules. For Polars Schema, to change the inferred Polars data types. |
 | [search](/src/cmd/search.rs#L2)<br>📇👆 | Run a regex over a CSV. Applies the regex to selected fields & shows only matching rows.  |
 | [searchset](/src/cmd/searchset.rs#L2)<br>📇👆 | _Run multiple regexes over a CSV in a single pass._ Applies the regexes to each field individually & shows only matching rows.  |
 | [select](/src/cmd/select.rs#L2)<br>👆 | Select, re-order, reverse, duplicate or drop columns.  |