Update linker README.

Change-Id: Icaa353e9cf1848c86e7445f4ad590bdab44f7941
diff --git a/linker/README.TXT b/linker/README.TXT
index 783b608..e75823e 100644
--- a/linker/README.TXT
+++ b/linker/README.TXT
@@ -79,8 +79,9 @@
 Debugging:
 ----------
 
-You can increase the verbosity of debug traces by defining the DEBUG
-environment variable to a numeric value from 0 to 2. This will only
+You can increase the verbosity of debug traces by defining the LD_DEBUG
+environment variable to a numeric value from 0 to 2 (corresponding to
+INFO, TRACE, and DEBUG calls in the source). This will only
 affect new processes being launched.
 
 By default, traces are sent to logcat, with the "linker" tag. You can