Add docs for cloning extensions repository (#32897)

Joseph T. Lyons and Peter Tripp created 

Release Notes:

- N/A

---------

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

Change summary

docs/src/extensions/developing-extensions.md | 18 ++++++++++++++++--
1 file changed, 16 insertions(+), 2 deletions(-)

Detailed changes

docs/src/extensions/developing-extensions.md 🔗 

@@ -89,12 +89,26 @@ impl zed::Extension for MyExtension {
 zed::register_extension!(MyExtension);
 ```
 
-## Publishing your extension
+## Forking and cloning the repo
 
-To publish an extension, open a PR to [the `zed-industries/extensions` repo](https://github.com/zed-industries/extensions).
+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).
+
 In your PR, do the following:
 
 1. Add your extension as a Git submodule within the `extensions/` directory