docs: fixes supabaseUrl is undefined error by adding missing step in DEVELOPERS.md (#20569)

docs: fixes supabaseUrl is undefined error by adding missing step

Co-authored-by: Chris Gwilliams <517923+encima@users.noreply.github.com>

DEVELOPERS.md

@@ -71,7 +71,13 @@ To contribute code to [Supabase](https://supabase.com), you must fork the [Supab
    npm install # install dependencies
    ```
 
-2. After that you can run the apps simultaneously with the following.
+2. Copy the example `.env.local.example` to `.env.local`
+
+   ```sh
+   cp apps/www/.env.local.example apps/www/.env.local
+   ```
+
+3. After that you can run the apps simultaneously with the following.
    ```sh
    npm run dev # start all the applications
    ```
@@ -92,6 +98,8 @@ You can run any of the sites individually by using the scope name. For example:
 npm run dev:www
 ```
 
+Note: Particularly for `www` make sure you have copied `apps/www/.env.local.example` to `apps/www/.env.local`
+
 #### Shared components
 
 The monorepo has a set of shared components under `/packages`: