add doc for downloading images

sample_data/README.md

@@ -0,0 +1,8 @@
+### Downloading images/sample data
+
+To download any file from Github, 
+If its an image, click on the file and you will then see a `Download` button on the right. If you click this, you can download the file. 
+
+![Downloading files](https://github.com/BioimageAnalysisCoreWEHI/napari_lattice/blob/master/resources/misc/download_files_github.jpg)
+
+If the image is `jpg` or `png`, you may see a preview of the image. If the file is not recognised by Github or if its really big to preview, then you'll see the window above.