Update doc about --bisect

diff --git a/README.md b/README.md
index 76a3010..5e2f7e4 100644
--- a/README.md
+++ b/README.md

@@ -125,7 +125,9 @@
 bazelisk --bisect=6.0.0..HEAD test //foo:bar_test
 ```
 
-### Useful environment variables
+Note that, Bazelisk uses prebuilt Bazel binaries at commits on the main and release branches, therefore you cannot bisect your local commits.
+
+### Useful environment variables for --migrate and --bisect
 
 You can set `BAZELISK_INCOMPATIBLE_FLAGS` to set a list of incompatible flags (separated by `,`) to be tested, otherwise Bazelisk tests all flags starting with `--incompatible_`.