Document running dev lightning on the cloud (#15962)

* document running dev lightning on the cloud

* document running dev lightning on the cloud

* Update .github/CONTRIBUTING.md

Co-authored-by: Noha Alon <nohalon@gmail.com>

* document running dev lightning on the cloud

* git clone & pip install -e

* Update .github/CONTRIBUTING.md

Co-authored-by: Jirka Borovec <6035284+Borda@users.noreply.github.com>

Co-authored-by: Noha Alon <nohalon@gmail.com>
Co-authored-by: Jirka Borovec <6035284+Borda@users.noreply.github.com>
(cherry picked from commit cfd00d3)

.github/CONTRIBUTING.md

@@ -309,6 +309,20 @@ and the last true master commit is `ccc111` and your first commit is `mmm222`.
   git push -f
   ```
 
+#### How to run an app on the cloud with a local version of lightning
+
+The lightning cloud uses the latest release by default. However, you might want to run your app with some local changes you've made to the lightning framework. To use your local version of lightning on the cloud, set the following environment variable:
+
+```bash
+git clone https://github.com/Lightning-AI/lightning.git
+cd lightning
+pip install -e .
+export PACKAGE_LIGHTNING=1  # <- this is the magic to use your version (not mainstream PyPI)!
+lightning run app app.py --cloud
+```
+
+By seting `PACKAGE_LIGHTNING=1`, lightning packages the lightning source code in your local directory in addition to your app source code and uploads them to the cloud.
+
 ### Bonus Workflow Tip
 
 If you don't want to remember all the commands above every time you want to push some code/setup a Lightning Dev environment on a new VM, you can set up bash aliases for some common commands. You can add these to one of your `~/.bashrc`, `~/.zshrc`, or `~/.bash_aliases` files.