Add a readme for display_test

examples/display_test/readme.md

@@ -0,0 +1,11 @@
+# display_test example
+
+This is a simple test of the cardputer's LCD. It draws a red rectangle which almost fills the display, leaving a 20 pixel border of black around the edge.
+
+## Run this example
+
+With this repo cloned, run the following from either the root or this directory:
+
+```shell
+cargo espflash flash --release --package display_test
+```