move some docs from L200

docs/source/index.rst

@@ -1,10 +1,47 @@
 Welcome to legendtools's documentation!
 ==========================================
 
+Registering detectors for use with `remage`_
+-----------------------------------------------
+
+On a physical volume instance, you can attach a ``pygeom_active_dector``, e.g.
+
+.. code:: python
+
+   from pygeomtools import detectors
+
+   pv = g4.PhysicalVolume(...)
+
+   # attach an active detector to this physical volume.
+   lv.pygeom_active_dector = detectors.RemageDetectorInfo(
+       "optical",  # detector type. The available options are defined by remage.
+       1,  # detector id in remage.
+       {"some": "metadata"},  # user-defined data (optional) that is stored into GDML.
+   )
+
+
+Adjusting the visualization
+---------------------------
+
+On a logical volume instance, you can set ``pygeom_color_rgba``, e.g.
+
+.. code:: python
+
+   lv = g4.LogicalVolume(...)
+
+   # hide this volume in the visualization
+   lv.pygeom_color_rgba = False
+
+   # set the vis coloring to the given RGBA value. All 4 numbers should be given in the range 0–1.
+   lv.pygeom_color_rgba = (r, g, b, a)
+
+
 Table of Contents
 -----------------
 
 .. toctree::
    :maxdepth: 1
 
    Package API reference <api/modules>
+
+.. _remage: https://github.com/legend-exp/remage