Update presubmit doc about requiring bazel version (#2366)

Fixes #2364

docs/README.md

@@ -79,7 +79,7 @@ Then the presubmit will verify building `@zlib//:zlib` succeeds on all specified
 
 While you can also specify `test_targets`, it may not always work since test targets can require additional dev dependencies that are not available when your project is not the root module.
 
-**Note that** the task config syntax follows [Bazel CI's specifications](https://github.com/bazelbuild/continuous-integration/tree/master/buildkite#configuring-a-pipeline).
+**Note that** the task config syntax follows [Bazel CI's specifications](https://github.com/bazelbuild/continuous-integration/tree/master/buildkite#configuring-a-pipeline). BCR requires the bazel version to be specified for each task via the `bazel` field.
 
 ### Test module
 
@@ -119,7 +119,7 @@ local_path_override(
 )
 ```
 
-**Note that** the task config syntax also follows [Bazel CI's specifications](https://github.com/bazelbuild/continuous-integration/tree/master/buildkite#configuring-a-pipeline), but just one level deeper under `bcr_test_module` and you have to specify the subdirectory of the test module via `module_path`.
+**Note that** the task config syntax also follows [Bazel CI's specifications](https://github.com/bazelbuild/continuous-integration/tree/master/buildkite#configuring-a-pipeline), but just one level deeper under `bcr_test_module` and you have to specify the subdirectory of the test module via `module_path`. BCR requires the bazel version to be specified for each task via the `bazel` field.
 
 ## Yank a module version