build: squash: explain node_modules exclusion in template

Co-authored-by: Feanil Patel <[email protected]>

workflow-templates/shellcheck.yml

@@ -23,6 +23,9 @@ jobs:
     with:
       # For details on the meaning of each of these arguments, see:
       # https://github.com/openedx/.github/blob/master/.github/workflows/shellcheck.yml
+      # We exclude `./node_modules/*` by default because we want people to easily be able to
+      # copy and run the command locally.  Local copies of most of our services have a `./node_modules` 
+      # directory that we want to ignore.
       exclude-patterns: "./node_modules/*"
       operating-system: "${{ matrix.os }}"
       #shellcheck-version: "v0.9.0"