ingest: Link to vendored/README.md for instructions on vendoring

Point users to the centralized usage docs of the vendored ingest repo
instead of maintaining a different copy of snippets that may become
out of sync.

README.md

@@ -153,13 +153,8 @@ aws s3 cp - s3://nextstrain-data/files/ncov/open/nextclade_21L.tsv.zst.renew < /
 
 ## `vendored`
 
-This repository uses [`git subrepo`](https://github.com/ingydotnet/git-subrepo) to manage copies of ingest scripts in `vendored`, from [nextstrain/ingest](https://github.com/nextstrain/ingest). To pull new changes from the central ingest repository, first install `git subrepo`, then run:
+This repository uses [`git subrepo`](https://github.com/ingydotnet/git-subrepo) to manage copies of ingest scripts in [`vendored`](./vendored), from [nextstrain/ingest](https://github.com/nextstrain/ingest). To pull new changes from the central ingest repository, first install `git subrepo`, then run:
 
-```sh
-git subrepo pull vendored
-```
-
-Changes should not be pushed using `git subrepo push`.
-
-1. For pathogen-specific changes, make them in this repository via a pull request.
-2. For pathogen-agnostic changes, make them on [nextstrain/ingest](https://github.com/nextstrain/ingest) via pull request there, then use `git subrepo pull` to add those changes to this repository.
+See [vendored/README.md](vendored/README.md#vendoring) for instructions on how to update
+the vendored scripts. Note that this repo is a special case and does not put vendored
+scripts in an `ingest/` directory. Modify commands accordingly.