update contributor docs to mention opam-repository pin upgrade process

Author: sabine

doc/FOR_MAINTAINERS.md

@@ -1,5 +1,14 @@
 # Information for maintainers
 
+# Advancing the `opam-repository` pin
+
+OCaml.org's dependencies are fixed through pinning `opam-repository` in the `dune-workspace` files, and in the `Dockerfile`. To update the repository pin,
+
+1. change the commit hash in the main `dune-workspace` file, `dune-workspace` file of the playground, and in the `Dockerfile`
+2. delete the `dune.lock/` directory
+3. build the project
+4. make it very obvious in the PR updating the opam-repository pin that contributors need to re-run `dune pkg lock` and be aware that contributors may need to be told to do this
+
 # How we maintain the staging branch
 
 To reduce the workload maintaining the `staging` branch in a good shape where