Add docs for updating Postgres for new minor versions

docs/updating-postgres.md

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+# Updating Postgres
+
+## Minor Versions
+
+When upgrading to a new minor version of Postgres, please follow these steps:
+
+_Example: 15.4 is the new minor version to upgrade to from 15.3._
+
+1. Clone the Neon Postgres repository if you have not done so already.
+
+    ```shell
+    git clone git@github.com:neondatabase/postgres.git
+    ```
+
+1. Add the Postgres upstream remote.
+
+    ```shell
+    git remote add upstream https://git.postgresql.org/git/postgresql.git
+    ```
+
+1. Create a new branch based on the stable branch you are updating.
+
+    ```shell
+    git checkout -b my-branch REL_15_STABLE_neon
+    ```
+
+1. Tag the last commit on the stable branch you are updating.
+
+    ```shell
+    git tag REL_15_3_neon
+    ```
+
+1. Push the new tag to the Neon Postgres repository.
+
+    ```shell
+    git push origin REL_15_3_neon
+    ```
+
+1. Find the release tags you're looking for. They are of the form `REL_X_Y`.
+
+1. Rebase the branch you created on the tag and resolve any conflicts.
+
+    ```shell
+    git fetch upstream REL_15_4
+    git rebase REL_15_4
+    ```
+
+1. Run the Postgres test suite to make sure our commits have not affected
+Postgres in a negative way.
+
+    ```shell
+    make check
+    # OR
+    meson test -C builddir
+    ```
+
+1. Push your branch to the Neon Postgres repository.
+
+    ```shell
+    git push origin my-branch
+    ```
+
+1. Clone the Neon repository if you have not done so already.
+
+    ```shell
+    git clone git@github.com:neondatabase/neon.git
+    ```
+
+1. Create a new branch.
+
+1. Change the `revisions.json` file to point at the HEAD of your Postgres
+branch.
+
+1. Update the Git submodule.
+
+    ```shell
+    git submodule set-branch --branch my-branch vendor/postgres-v15
+    git submodule update --remote vendor/postgres-v15
+    ```
+
+1. Run the Neon test suite to make sure that Neon is still good to go on this
+minor Postgres release.
+
+    ```shell
+    ./scripts/poetry -k pg15
+    ```
+
+1. Commit your changes.
+
+1. Create a pull request, and wait for CI to go green.
+
+1. Force push the rebased Postgres branches into the Neon Postgres repository.
+
+    ```shell
+    git push --force origin my-branch:REL_15_STABLE_neon
+    ```
+
+    It may require disabling various branch protections.
+
+1. Update your Neon PR to point at the branches.
+
+    ```shell
+    git submodule set-branch --branch REL_15_STABLE_neon vendor/postgres-v15
+    git commit --amend --no-edit
+    git push --force origin
+    ```
+
+1. Merge the pull request after getting approval(s) and CI completion.