Add docs and README entries for derived morphological operators (#1025)

Author: brendancol

README.md

@@ -181,6 +181,9 @@ In the GIS world, rasters are used for representing continuous phenomena (e.g. e
 | [Dilate](xrspatial/morphology.py) | Morphological dilation (local maximum over structuring element) | Standard (morphology) | ✅️ | ✅️ | ✅️ | ✅️ |
 | [Opening](xrspatial/morphology.py) | Erosion then dilation (removes small bright features) | Standard (morphology) | ✅️ | ✅️ | ✅️ | ✅️ |
 | [Closing](xrspatial/morphology.py) | Dilation then erosion (fills small dark gaps) | Standard (morphology) | ✅️ | ✅️ | ✅️ | ✅️ |
+| [Gradient](xrspatial/morphology.py) | Dilation minus erosion (edge detection) | Standard (morphology) | ✅️ | ✅️ | ✅️ | ✅️ |
+| [White Top-hat](xrspatial/morphology.py) | Original minus opening (isolate bright features) | Standard (morphology) | ✅️ | ✅️ | ✅️ | ✅️ |
+| [Black Top-hat](xrspatial/morphology.py) | Closing minus original (isolate dark features) | Standard (morphology) | ✅️ | ✅️ | ✅️ | ✅️ |
 
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docs/source/reference/morphology.rst

@@ -32,6 +32,27 @@ Closing
 
    xrspatial.morphology.morph_closing
 
+Gradient
+========
+.. autosummary::
+   :toctree: _autosummary
+
+   xrspatial.morphology.morph_gradient
+
+White top-hat
+=============
+.. autosummary::
+   :toctree: _autosummary
+
+   xrspatial.morphology.morph_white_tophat
+
+Black top-hat
+=============
+.. autosummary::
+   :toctree: _autosummary
+
+   xrspatial.morphology.morph_black_tophat
+
 Kernel Construction
 ===================
 .. autosummary::