Fix build status badge to show status from master only. (#299)

Failures in other branches / PRs will otherwise cause it to turn red. :)
diff --git a/README.md b/README.md
index e11d216..1d61048 100644
--- a/README.md
+++ b/README.md
@@ -1,6 +1,6 @@
 Bazel CI
 :---:
-[![Build status](https://badge.buildkite.com/940075452c1c5ff91dc832664c4c8f05e6ec736916688cd894.svg)](https://buildkite.com/bazel/bazel-toolchains-postsubmit)
+[![Build status](https://badge.buildkite.com/940075452c1c5ff91dc832664c4c8f05e6ec736916688cd894.svg?branch=master)](https://buildkite.com/bazel/bazel-toolchains-postsubmit)
 
 # bazel-toolchains
 
@@ -22,4 +22,4 @@
 This repo previously contained Bazel targets that are used to generate toolchain
 containers. Note that they have been migrated to the following two repos:
 * [layer-definitions](https://github.com/GoogleCloudPlatform/layer-definitions)
-* [container-definitions](https://github.com/GoogleCloudPlatform/container-definitions)
\ No newline at end of file
+* [container-definitions](https://github.com/GoogleCloudPlatform/container-definitions)