docs: update

.github/workflows/main.yml

@@ -0,0 +1,9 @@
+name: Shellcheck
+on: [push]
+jobs:
+    build:
+        runs-on: ubuntu-latest
+        steps:
+            - uses: actions/checkout@v1
+            - name: Run shellcheck.
+              run: ./test.sh

test.sh

@@ -0,0 +1,24 @@
+#!/bin/sh
+# shellcheck source=/dev/null
+#
+# Tests for the Pure sh Bible.
+
+main() {
+    trap 'rm -f readme_code test_file' EXIT INT
+
+    # Extract code blocks from the README.
+    while read -r line; do
+        [ "$code" ] && [ "$line" != \`\`\` ] &&
+            printf '%s\n' "$line"
+
+        case $line in
+            \`\`\`sh) code=1 ;;
+            \`\`\`)   code=
+        esac
+    done < README.md > readme_code
+
+    # Run shellcheck and source the code.
+    shellcheck -s sh readme_code test.sh || exit 1
+}
+
+main "$@"