adds kokoro presubmit file (GoogleCloudPlatform#595)

Author: bshaffer

.kokoro/presubmit.cfg

@@ -0,0 +1,10 @@
+# Format: //devtools/kokoro/config/proto/build.proto
+
+# Download secrets from Cloud Storage.
+gfile_resources: "/bigstore/cloud-devrel-kokoro-resources/php-docs-samples"
+
+# Tell the trampoline which build file to use.
+env_vars: {
+    key: "TRAMPOLINE_BUILD_FILE"
+    value: "github/php-docs-samples/.kokoro/system_tests.sh"
+}