DOC: add release note entry for the elision

doc/release/1.13.0-notes.rst

@@ -7,6 +7,8 @@ This release supports Python 2.7 and 3.4 - 3.6.
 Highlights
 ==========
 
+ * Operations like `a + b + c` will create less temporaries on some platforms
+
 
 Dropped Support
 ===============
@@ -84,6 +86,14 @@ C API
 New Features
 ============
 
+Temporary elision
+-----------------
+On platforms providing the `backtrace` function NumPy will now not create
+temporaries in expression when possible.
+For example `d = a + b + c` is transformed to `d = a + b; d += c` which can
+improve performance for large arrays as less memory bandwidth is required to
+perform the operation.
+
 ``axes`` argument for ``unique``
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 In an N-dimensional array, the user can now choose the axis along which to look