Update "smoke test" examples compilation CI workflow

On every push or pull request that affects library source or example files, and periodically, compile all example
sketches for the specified boards.

Author: per1234

.github/workflows/compile-examples.yml

@@ -0,0 +1,52 @@
+name: Compile Examples
+
+# See: https://docs.github.com/en/free-pro-team@latest/actions/reference/events-that-trigger-workflows
+on:
+  push:
+    paths:
+      - ".github/workflows/compile-examples.yml"
+      - "examples/**"
+      - "src/**"
+  pull_request:
+    paths:
+      - ".github/workflows/compile-examples.yml"
+      - "examples/**"
+      - "src/**"
+  schedule:
+    # Run every Tuesday at 8 AM UTC to catch breakage caused by changes to external resources (libraries, platforms).
+    - cron: "0 8 * * TUE"
+  workflow_dispatch:
+  repository_dispatch:
+
+jobs:
+  build:
+    name: ${{ matrix.board.fqbn }}
+    runs-on: ubuntu-latest
+
+    strategy:
+      fail-fast: false
+
+      matrix:
+        board:
+          - fqbn: esp8266:esp8266:huzzah
+            platforms: |
+              - name: esp8266:esp8266
+                source-url: https://arduino.esp8266.com/stable/package_esp8266com_index.json
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v2
+
+      - name: Compile examples
+        uses: arduino/compile-sketches@v1
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+          fqbn: ${{ matrix.board.fqbn }}
+          platforms: ${{ matrix.board.platforms }}
+          libraries: |
+            # Install the library from the local path.
+            - source-path: ./
+            # Additional library dependencies can be listed here.
+            # See: https://github.com/arduino/compile-sketches#libraries
+          sketch-paths: |
+            - examples

.travis.yml

@@ -1,6 +1,6 @@
 # NTPClient
 
-[![Build Status](https://travis-ci.org/arduino-libraries/NTPClient.svg?branch=master)](https://travis-ci.org/arduino-libraries/NTPClient)
+[![Compile Examples status](https://github.com/arduino-libraries/NTPClient/actions/workflows/compile-examples.yml/badge.svg)](https://github.com/arduino-libraries/NTPClient/actions/workflows/compile-examples.yml)
 
 Connect to a NTP server, here is how: