I. natcap#1104 adding note to history

HISTORY.rst

@@ -41,6 +41,17 @@ Unreleased Changes
       ``sphinx-rtd-theme`` and ``sphinx-reredirects`` will be pulled from
       conda-forge instead of PyPI.
       (`#1151 <https://github.com/natcap/invest/issues/1151>`_)
+* Habitat Quality
+    * The model now uses an euclidean distance implementation for decaying
+      threat rasters both linearly and exponentially. Since InVEST 3.3.0 a
+      convolution implemenation has been used, which reflected how
+      the density of a threat or surrounding threat pixels could have an
+      even greater, cumulative impact and degradation over space. However, this
+      was never properly documented in the User's Guide and is not the approach
+      taken in the publication. The convolution implementation also produced
+      degradation and quality outputs that were difficult to interpret.
+    * There should be a noticeable runtime improvement from calculating
+      euclidean distances vs convolutions.
 * Workbench
     * Added tooltips to the model tabs so that they can be identified even when
       several tabs are open (`#1071 <https://github.com/natcap/invest/issues/1088>`_)