Add docs for cloning extensions repository (#32897)

Release Notes:

- N/A

---------

Co-authored-by: Peter Tripp <peter@zed.dev>

docs/src/extensions/developing-extensions.md

@@ -89,12 +89,26 @@ impl zed::Extension for MyExtension {
 zed::register_extension!(MyExtension);
 ```
 
+## Forking and cloning the repo
+
+1. Fork the repo
+
+> Note: It is very helpful if you fork the `zed-industries/extensions` repo to a personal GitHub account instead of a GitHub organization, as this allows Zed staff to push any needed changes to your PR to expedite the publishing process.
+
+2. Clone the repo to your local machine
+
+```sh
+# Substitute the url of your fork here:
+# git clone https://github.com/zed-industries/extensions
+cd extensions
+git submodule init
+git submodule update
+```
+
 ## Publishing your extension
 
 To publish an extension, open a PR to [the `zed-industries/extensions` repo](https://github.com/zed-industries/extensions).
 
-> Note: It is very helpful if you fork the `zed-industries/extensions` repo to a personal GitHub account instead of a GitHub organization, as this allows Zed staff to push any needed changes to your PR to expedite the publishing process.
-
 In your PR, do the following:
 
 1. Add your extension as a Git submodule within the `extensions/` directory