add screenshots folder AND a track on how to use it

vignettes/screenshots/README.md

@@ -0,0 +1,19 @@
+# Vignette screenshots
+
+This folder contains the screenshots of the app, generated in a pseudo-automated manner - one that does not involve too much of cropping, resizing, and customizing resolution.
+
+The way it works:
+
+```
+app <- iSEE(sce, initial)   # as in the vignette chunk 
+```
+
+Once you created the `app` object, call `appshot()` from the `webshot2` package
+
+```
+webshot2::appshot(app, file = "screenshots/app_snapped.png", delay = 20)
+```
+
+The use of the delay parameter is to ensure that the app has loaded all the panels correctly.
+
+Then, in the vignette, we simply use `knitr::include_graphics()` to embed the images as we normally would.