add link checker only for README.md

Author: spier

.github/workflows/main.yml

@@ -0,0 +1,17 @@
+# from: https://github.com/marketplace/actions/link-checker
+
+name: Link Check on README.md
+
+on: push
+jobs:
+  linkChecker:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - name: Link Checker
+      id: lc
+      uses: peter-evans/link-checker@v1
+      with:
+        args: -v -d . -x http://creativecommons.org/licenses README.md
+    - name: Fail if there were link errors
+      run: exit ${{ steps.lc.outputs.exit_code }}