docs: update joinp description to add --maintain-order option

README.md

@@ -57,7 +57,7 @@
 | [index](/src/cmd/index.rs#L2) | Create an index (📇) for a CSV. This is very quick (even the 15gb, 28m row NYC 311 dataset takes all of 14 seconds to index) & provides constant time indexing/random access into the CSV. With an index, `count`, `sample` & `slice` work instantaneously; random access mode is enabled in `luau`; and multithreading (🏎️) is enabled for the `frequency`, `split`, `stats`, `schema` & `tojsonl` commands. |
 | [input](/src/cmd/input.rs#L2) | Read CSV data with special commenting, quoting, trimming, line-skipping & non-UTF8 encoding handling rules. Typically used to "normalize" a CSV for further processing with other qsv commands. |
 | [join](/src/cmd/join.rs#L2)<br>👆 | Inner, outer, right, cross, anti & semi joins. Automatically creates a simple, in-memory hash index to make it fast.  |
-| [joinp](/src/cmd/joinp.rs#L2)✨<br>🚀🐻‍❄️🪄 | Inner, outer, right, cross, anti, semi & asof joins using the [Pola.rs](https://www.pola.rs) engine. Unlike the `join` command, `joinp` can process files larger than RAM, is multithreaded, has join key validation, pre-join filtering, supports [asof joins](https://pola-rs.github.io/polars/py-polars/html/reference/dataframe/api/polars.DataFrame.join_asof.html) (which is [particularly useful for time series data](https://github.com/dathere/qsv/blob/30cc920d0812a854fcbfedc5db81788a0600c92b/tests/test_joinp.rs#L509-L983)) & its output columns can be coalesced. |
+| [joinp](/src/cmd/joinp.rs#L2)✨<br>🚀🐻‍❄️🪄 | Inner, outer, right, cross, anti, semi & asof joins using the [Pola.rs](https://www.pola.rs) engine. Unlike the `join` command, `joinp` can process files larger than RAM, is multithreaded, has join key validation, a maintain row order option, pre-join filtering, supports [asof joins](https://pola-rs.github.io/polars/py-polars/html/reference/dataframe/api/polars.DataFrame.join_asof.html) (which is [particularly useful for time series data](https://github.com/dathere/qsv/blob/30cc920d0812a854fcbfedc5db81788a0600c92b/tests/test_joinp.rs#L509-L983)) & its output columns can be coalesced. |
 | [json](/src/cmd/json.rs#L2)<br>👆 | Convert JSON to CSV.
 | [jsonl](/src/cmd/jsonl.rs#L2)<br>🚀🔣 | Convert newline-delimited JSON ([JSONL](https://jsonlines.org/)/[NDJSON](http://ndjson.org/)) to CSV. See `tojsonl` command to convert CSV to JSONL.
 | [lens](/src/cmd/lens.rs#L2)✨ | Interactively view, search & filter a CSV using the [csvlens](https://github.com/YS-L/csvlens#csvlens) engine.