document engine-p porting (#3868)

* document engine-p porting

See #3390 for all the details.

* use static numbering

docs/howto_port_a_command_to_engine_p.md

@@ -0,0 +1,39 @@
+# How To Port Old Engine Syntax to the Latest and Greatest
+
+## engine-p
+
+Even in the new codebase of nu things change and we have some old code to clean up.
+This guide walks you through how to port old engine syntax to engine-p.
+
+1. change into the commands source directory
+
+```sh
+cd crates/nu-command/src/
+```
+
+2. search for old syntax using ripgrep (`rg`)
+
+```sh
+rg --type rust --files-with-matches 'ActionStream|run_with_actions'
+```
+
+3. update the old syntax engine-p syntax
+  - For a smaller example PR see [#3794](https://github.com/nushell/nushell/pull/3794/files)
+  - For a more involved example PR see [#3649](https://github.com/nushell/nushell/pull/3649/files)
+
+In many cases this is changing the function name and signature.
+The function name goes from `run_with_actions` to `run`.
+The signature goes from `Result<ActionStream, ShellError>`
+to `Result<OutputStream, ShellError>`.
+
+Sometimes you cannot alter the signature because the command
+still requires working with `ActionStream`.
+[open](https://github.com/nushell/nushell/blob/28db8022fef7c1f7f44f6e50d07d3f42773deddf/crates/nu-command/src/commands/filesystem/open.rs) is one example.
+
+4. commit changes, run the tests, filtered by the command package and section name:
+
+```sh
+cargo test --features=extra -p nu-command string
+```
+
+5. PR and bask in the glory of the nu-and-improved codebase.