docs for RAISE_SUBPROC_ERROR

CHANGELOG.rst

@@ -5,7 +5,12 @@ Xonsh Change Log
 Current Developments
 ====================
 
-**Added:** None
+**Added:** 
+
+* Added boolean ``$RAISE_SUBPROC_ERROR`` environment variable. If true
+  and a subprocess command exits with a non-zero return code, a 
+  CalledProcessError will be raised. This is useful in scripts that should
+  fail at the first error.
 
 **Changed:** None
 

docs/tutorial.rst

@@ -1018,6 +1018,11 @@ operates on a given argument, rather than on the string ``'xonsh'`` (notice how
     bash $ echo @(' '.join($(cat @('file%d.txt' % i)).strip() for i in range(6)))
     s n a i l s
 
+Additionally, if the script should exit if a command fails, set the 
+environment variable ``$RAISE_SUBPROC_ERROR = True`` at the top of the 
+file. Errors in Python mode will already raise exceptions and so this 
+is roughly equivelent to Bash's `set -e`.
+
 Importing Xonsh (``*.xsh``)
 ==============================
 You can import xonsh source files with the ``*.xsh`` file extension using