produce a README

Author: jepler

.github/workflows/githubci.yml

@@ -51,7 +51,12 @@ jobs:
       if: startsWith(github.ref, 'refs/tags/')
       run: |
         if [ -d build ]; then
-            cd build && zip -9 -o ${{ matrix.arduino-platform }}.zip *.hex *.bin *.uf2
+            (
+            echo "Built from Adafruit Learning System Guides `git describe --tags` for ${{ matrix.arduino-platform }}"
+            echo "Source code: https://github.com/adafruit/"
+            echo "Adafruit Learning System: https://learn.adafruit.com/"
+            ) > build/README.txt
+            cd build && zip -9 -o ${{ matrix.arduino-platform }}.zip *.hex *.bin *.uf2 *.txt
         fi
 
     - name: Create release