Add CI workflow to check for commonly misspelled words

In the event of a false positive, the problematic word should be added, in all lowercase, to
extras/codespell-ignore-words-list.txt, after which it will be ignored by the action.

Author: per1234

.github/workflows/spell-check.yml

@@ -0,0 +1,26 @@
+name: Spell Check
+
+on:
+  pull_request:
+  push:
+  schedule:
+    # run every Tuesday at 3 AM UTC
+    - cron: "0 3 * * 2"
+
+jobs:
+  spellcheck:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+
+      # See: https://github.com/codespell-project/actions-codespell/blob/master/README.md
+      - name: Spell check
+        uses: codespell-project/actions-codespell@master
+        with:
+          check_filenames: true
+          check_hidden: true
+          # In the event of a false positive, add the word in all lower case to this file:
+          ignore_words_file: extras/codespell-ignore-words-list.txt
+          skip: ./.git,./src/libmodbus

README.adoc

@@ -5,6 +5,7 @@
 = Modbus Library for Arduino =
 
 image:https://github.com/{repository-owner}/{repository-name}/workflows/Compile%20Examples/badge.svg["Compile Examples Status", link="https://github.com/{repository-owner}/{repository-name}/actions?workflow=Compile+Examples"]
+image:https://github.com/{repository-owner}/{repository-name}/workflows/Spell%20Check/badge.svg["Spell Check Status", link="https://github.com/{repository-owner}/{repository-name}/actions?workflow=Spell+Check"]
 
 Use http://www.modbus.org/[Modbus] with your Arduino.