Add comments for ANDROID_BUILD_SHELL

Change-Id: I71f0a1a08b097cd179bbcb684a9114267f9bf0db
diff --git a/core/main.mk b/core/main.mk
index 3da799b..7c1ff12 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -1,3 +1,5 @@
+# Only use ANDROID_BUILD_SHELL to wrap around bash.
+# DO NOT use other shells such as zsh.
 ifdef ANDROID_BUILD_SHELL
 SHELL := $(ANDROID_BUILD_SHELL)
 else