Explain how to add new examples to the gallery (napari#137)

# Description
Adds some explanation on how to add new examples to the gallery.

## Type of change
<!-- Please delete options that are not relevant. -->
- [x] Fixes or improves existing content

# References
Closes napari#92

## Final checklist:
- [x] My PR is the minimum possible work for the desired functionality
---------

Co-authored-by: Peter Sobolewski <76622105+psobolewskiPhD@users.noreply.github.com>

docs/developers/contributing.md

@@ -112,6 +112,63 @@ Now you are all set to start developing with napari.
 
 If you wish to contribute documentation changes to napari, please read the [guide on contributing documentation](documentation/index.md). 
 
+(add-examples)=
+## Adding examples to the [Gallery](gallery)
+
+All of the examples in the [examples Gallery](gallery) are Python scripts that
+live under [the `examples` folder of the napari repository](https://github.com/napari/napari/tree/main/examples).
+Because of how this gallery is built, every such Python script needs a docstring
+containing at least a title (following the `.rst` format of docstrings in
+Python) and one or more _tags_. This ensures the example will be shown in the
+correct place in the table of contents for the gallery (see, for example, the
+[](tagoverview) page.)
+
+Here's an example of what that means. The example file
+[add_image.py](https://github.com/napari/napari/blob/main/examples/add_image.py)
+contains the following:
+
+```python
+"""
+Add image
+=========
+
+Display one image using the :func:`view_image` API.
+
+.. tags:: visualization-basic
+"""
+
+from skimage import data
+import napari
+
+# create the viewer with an image
+viewer = napari.view_image(data.astronaut(), rgb=True)
+
+if __name__ == '__main__':
+    napari.run()
+```
+
+The equals signs under "Add image" mark this as the example title. The text
+below is a short description of the example, and the tags are assigned by the
+use of the `.. tags::` directive. In this case, the only tag associated with
+this example is `visualization-basic`. Multiple tags can be separated with a
+comma.
+
+Note that the examples are `.py` source files, and any outputs and images will
+be autogenerated when the documentation site is built.
+
+````{note}
+If you are running any of these examples in an interactive environment (e.g. a Jupyter notebook or
+IPython console), the block
+
+```
+if __name__ == '__main__':
+    napari.run()
+```
+
+is not necessary—you can copy-paste the code above it. However, it is required when running examples as scripts, using for example `python add_image.py`.
+Because our example gallery is built from Python scripts, you need to ensure this
+block is present in all contributed examples.
+````
 ## Adding icons
 
 If you want to add a new icon to the app, make the icon in whatever program you

docs/developers/documentation/index.md

@@ -11,7 +11,8 @@ The napari documentation is built from sources located at the
 repository is where all the narrative documentation (e.g. tutorials, how-to
 guides) pull requests should be made. Meanwhile, changes to docstrings or to the
 [examples gallery](https://napari.org/gallery) should be made to the
-[napari/napari](https://github.com/napari/napari) repository.
+[napari/napari](https://github.com/napari/napari) repository (see also
+[](add-examples)).
 
 ## Prerequisites
 

docs/gallery.md

@@ -3,6 +3,12 @@
 
 Examples of napari usage.
 
+```{note}
+
+If you want to submit a new example to this gallery, check out our
+[contribution guide](add-examples)!
+```
+
 ```{toctree}
 :maxdepth: 1