README: Document workaround with subrepo's --force flag

In walking through `git subrepo pull` with @j23414 for monkeypox, we
ran into unexpected merge conflicts.¹

Reading through the git subrepo docs,² I found that we can just make a
clean "reclone" of the subrepo with the `--force` flag. Instead of
working through the conflict resolution, the forced reclone made sense
for just getting the latest version of the ingest repo.

¹ https://gist.github.com/joverlee521/b00739c3dcbff66ca99828d43cb80a42
² https://github.com/ingydotnet/git-subrepo#commands

README.md

@@ -25,6 +25,13 @@ Any future updates of ingest scripts can be pulled in with:
 git subrepo pull ingest/vendored
 ```
 
+If you run into merge conflicts and would like to pull in a fresh copy of the
+latest ingest scripts, pull with the `--force` flag:
+
+```
+git subrepo pull ingest/vendored --force
+```
+
 > **Warning**
 > Beware of rebasing/dropping the parent commit of a `git subrepo` update