Add NEWS item for pipe rewrite

NEWS.md

@@ -1,5 +1,33 @@
+
 # magrittr 1.5.0.9000
 
+## Fast and lean implementation of the pipe
+
+The pipe has been rewritten in C.
+
+- Minimal performance cost.
+- Minimal impact on backtraces.
+- No impact on reference counts.
+
+As part of this rewrite we have slightly changed the behaviour of the
+pipe so that the piped expressions are now evaluated in the current
+environment. Previously, the pipe evaluated in its own private
+environment where `.` was defined. This is technically a breaking
+change, but this should only affect very specific corner cases and it
+brings the behaviour of the pipe closer to other control flow
+mechanisms like `if ()` or `for ()` which also evaluate in the current
+environment. This also brings it closer to the probable semantics of
+the native R pipe that is likely to be introduced in the next version
+of R. The most visible consequences of this new behaviour are:
+
+- `parent.frame()` now returns the same environment in piped and
+  non-piped evaluation (#146, #171).
+
+- `return()` returns from the enclosing function. It would previously
+  return from the current pipe expression and continue evaluation from
+  there.
+
+
 # magrittr 1.5
 
 ## New features