Clarifying how to disable 'crash_log'
The current set of instructions mention 'crash_log' being left undefined
as a variable to disable it, but when that happens, lager starts it at a
default location.
This commit makes it obvious that the value must explicitly set to
'undefined' in order to disable crash_log.
diff --git a/README.md b/README.md
index 2d2758c..e3475ad 100644
--- a/README.md
+++ b/README.md
@@ -137,7 +137,7 @@
The error_logger handler will also log more complete error messages (protected
with use of trunc_io) to a "crash log" which can be referred to for further
information. The location of the crash log can be specified by the crash_log
-application variable. If undefined it is not written at all.
+application variable. If set to `undefined` it is not written at all.
Messages in the crash log are subject to a maximum message size which can be
specified via the crash_log_msg_size application variable.
