Add CI workflow to check for commonly misspelled words

On every push, pull request, and periodically, use the codespell-project/actions-codespell action to check for commonly
misspelled words.

In the event of a false positive, the problematic word should be added, in all lowercase, to the ignore-words-list field
of ./.codespellrc. Regardless of the case of the word in the false positive, it must be in all lowercase in the ignore
list. The ignore list is comma-separated with no spaces.

The URLs in the workflow status badge are configured for the final location of the library, so they are expected to be
broken while it is in the temporary development location.

Author: per1234

.codespellrc

@@ -0,0 +1,8 @@
+# See: https://github.com/codespell-project/codespell#using-a-config-file
+[codespell]
+# In the event of a false positive, add the problematic word, in all lowercase, to a comma-separated list here:
+ignore-words-list = inot
+skip = ./.git
+builtin = clear,informal,en-GB_to_en-US
+check-filenames =
+check-hidden =

.github/workflows/spell-check.yml

@@ -0,0 +1,22 @@
+name: Spell Check
+
+# See: https://docs.github.com/en/actions/reference/events-that-trigger-workflows
+on:
+  push:
+  pull_request:
+  schedule:
+    # Run every Tuesday at 8 AM UTC to catch new misspelling detections resulting from dictionary updates.
+    - cron: "0 8 * * TUE"
+  workflow_dispatch:
+  repository_dispatch:
+
+jobs:
+  spellcheck:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v2
+
+      - name: Spell check
+        uses: codespell-project/actions-codespell@master

README.md

@@ -2,5 +2,6 @@
 
 [![Compile Examples status](https://github.com/arduino-libraries/Arduino_Threads/actions/workflows/compile-examples.yml/badge.svg)](https://github.com/arduino-libraries/Arduino_Threads/actions/workflows/compile-examples.yml)
 [![Check Arduino status](https://github.com/arduino-libraries/Arduino_Threads/actions/workflows/check-arduino.yml/badge.svg)](https://github.com/arduino-libraries/Arduino_Threads/actions/workflows/check-arduino.yml)
+[![Spell Check status](https://github.com/arduino-libraries/Arduino_Threads/actions/workflows/spell-check.yml/badge.svg)](https://github.com/arduino-libraries/Arduino_Threads/actions/workflows/spell-check.yml)
 
 This library makes it easy to use the multi-threading capability of [Arduino](https://www.arduino.cc/) boards that use an [Mbed OS](https://os.mbed.com/docs/mbed-os/latest/introduction/index.html)-based core library.