Add CI workflow to check the license file

Whenever one of the recognized license file names are modified in the repository, the workflow runs to check whether the
license can be recognized and whether it is of the expected type.

GitHub has a useful automated license detection system that determines the license type used by a repository, and
surfaces that information in the repository home page, the search web interface, and the GitHub API. This license
detection system requires that the license be defined by a dedicated file with one of several standardized filenames and
paths.

GitHub's license detection system uses the popular licensee tool, so this file also serves to define the license type
for any other usages of licensee, as well as to human readers of the file.

For this reason, and to ensure it remains a valid legal instrument, it's important that there be no non-standard
modifications to the license file or collisions with other supported licence files. This workflow ensures that any
changes which would change the license type or which license file is used by the detection are caught automatically.

.github/workflows/check-license.yml

@@ -0,0 +1,67 @@
+# Source: https://github.com/arduino/tooling-project-assets/blob/main/workflow-templates/check-license.md
+name: Check License
+
+env:
+  EXPECTED_LICENSE_FILENAME: LICENSE.txt
+  # SPDX identifier: https://spdx.org/licenses/
+  EXPECTED_LICENSE_TYPE: CC0-1.0
+
+# See: https://docs.github.com/en/actions/reference/events-that-trigger-workflows
+on:
+  push:
+    paths:
+      - ".github/workflows/check-license.ya?ml"
+      # See: https://github.com/licensee/licensee/blob/master/docs/what-we-look-at.md#detecting-the-license-file
+      - "[cC][oO][pP][yY][iI][nN][gG]*"
+      - "[cC][oO][pP][yY][rR][iI][gG][hH][tH]*"
+      - "[lL][iI][cC][eE][nN][cCsS][eE]*"
+      - "[oO][fF][lL]*"
+      - "[pP][aA][tT][eE][nN][tT][sS]*"
+  pull_request:
+    paths:
+      - ".github/workflows/check-license.ya?ml"
+      - "[cC][oO][pP][yY][iI][nN][gG]*"
+      - "[cC][oO][pP][yY][rR][iI][gG][hH][tH]*"
+      - "[lL][iI][cC][eE][nN][cCsS][eE]*"
+      - "[oO][fF][lL]*"
+      - "[pP][aA][tT][eE][nN][tT][sS]*"
+  workflow_dispatch:
+  repository_dispatch:
+
+jobs:
+  check-license:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v2
+
+      - name: Install Ruby
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: ruby # Install latest version
+
+      - name: Install licensee
+        run: gem install licensee
+
+      - name: Check license file
+        run: |
+          EXIT_STATUS=0
+          # See: https://github.com/licensee/licensee
+          LICENSEE_OUTPUT="$(licensee detect --json --confidence=100)"
+
+          DETECTED_LICENSE_FILE="$(echo "$LICENSEE_OUTPUT" | jq .matched_files[0].filename | tr --delete '\r')"
+          echo "Detected license file: $DETECTED_LICENSE_FILE"
+          if [ "$DETECTED_LICENSE_FILE" != "\"${EXPECTED_LICENSE_FILENAME}\"" ]; then
+            echo "::error file=${DETECTED_LICENSE_FILE}::detected license file $DETECTED_LICENSE_FILE doesn't match expected: $EXPECTED_LICENSE_FILENAME"
+            EXIT_STATUS=1
+          fi
+
+          DETECTED_LICENSE_TYPE="$(echo "$LICENSEE_OUTPUT" | jq .matched_files[0].matched_license | tr --delete '\r')"
+          echo "Detected license type: $DETECTED_LICENSE_TYPE"
+          if [ "$DETECTED_LICENSE_TYPE" != "\"${EXPECTED_LICENSE_TYPE}\"" ]; then
+            echo "::error file=${DETECTED_LICENSE_FILE}::detected license type $DETECTED_LICENSE_TYPE doesn't match expected \"${EXPECTED_LICENSE_TYPE}\""
+            EXIT_STATUS=1
+          fi
+
+          exit $EXIT_STATUS

README.md

@@ -9,6 +9,7 @@
 [![Check Taskfiles status](https://github.com/arduino/library-registry/actions/workflows/check-taskfiles.yml/badge.svg)](https://github.com/arduino/library-registry/actions/workflows/check-taskfiles.yml)
 [![Check Workflows status](https://github.com/arduino/library-registry/actions/workflows/check-workflows-task.yml/badge.svg)](https://github.com/arduino/library-registry/actions/workflows/check-workflows-task.yml)
 [![Check YAML status](https://github.com/arduino/library-registry/actions/workflows/check-yaml-task.yml/badge.svg)](https://github.com/arduino/library-registry/actions/workflows/check-yaml-task.yml)
+[![Check License status](https://github.com/arduino/library-registry/actions/workflows/check-license.yml/badge.svg)](https://github.com/arduino/library-registry/actions/workflows/check-license.yml)
 
 This branch contains the generated source file for the Arduino Library Manager index. If you want to add a library to
 the Arduino Library Manager index, please follow these instructions: