tests.nixpkgs-check-by-name: Add documentation for scripts

pkgs/test/nixpkgs-check-by-name/scripts/README.md

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+# CI-related Scripts
+
+This directory contains scripts used and related to the CI running the `pkgs/by-name` checks in Nixpkgs. See also the [CI GitHub Action](../../../../.github/workflows/check-by-name.yml).
+
+## `./run-local.sh BASE_BRANCH [REPOSITORY]`
+
+Runs the `pkgs/by-name` check on the HEAD commit, closely matching what CI does.
+
+Note that this can't do exactly the same as CI,
+because CI needs to rely on GitHub's server-side Git history to compute the mergeability of PRs before the check can be started.
+In turn when running locally, we don't want to have to push commits to test them,
+and we can also rely on the local Git history to do the mergeability check.
+
+Arguments:
+- `BASE_BRANCH`: The base branch to use, e.g. master or release-23.11
+- `REPOSITORY`: The repository to fetch the base branch from, defaults to https://github.com/NixOS/nixpkgs.git
+
+## `./fetch-tool.sh BASE_BRANCH OUTPUT_PATH`
+
+Fetches the Hydra-prebuilt nixpkgs-check-by-name to use from the NixOS channel corresponding to the given base branch.
+
+This script is used both by [`./run-local.sh`](#run-local-sh-base-branch-repository) and CI.
+
+Arguments:
+- `BASE_BRANCH`: The base branch to use, e.g. master or release-23.11
+- `OUTPUT_PATH`: The output symlink path for the tool