Handle submodules with SSH URLs

This is just a documentation change, explaining how to fix submodules
that are configured to use SSH URLs instead of HTTPS URLs. Spent a while
banging my head on the wall and hope this saves someone else the pain.

This is helpful for teams that use the SSH protocol for local
development so don't want to change the mechanism that pulls in the
submodules. Using `insteadOf` seems a bit nicer than than setting up a
deploy keypair.

README.md

@@ -180,6 +180,7 @@ jobs:
 - name: Checkout submodules
   shell: bash
   run: |
+    git config --global url."https://github.com/".insteadOf "git@github.com:"
     auth_header="$(git config --local --get http.https://github.com/.extraheader)"
     git submodule sync --recursive
     git -c "http.extraheader=$auth_header" -c protocol.version=2 submodule update --init --force --recursive --depth=1