DOC: Describe speclite filters in documentation (#457)

Author: philipp128

docs/galaxies.rst

@@ -63,6 +63,13 @@ Spectrum
 
 The following models are found in the `skypy.galaxies.spectrum` package.
 
+SkyPy uses the `speclite <https://speclite.readthedocs.io/>`_ package for
+photometric calculations. Some of the following functions take the names of
+photometric filters as an input parameter. Users can choose from the available
+`Speclite Filters <https://speclite.readthedocs.io/en/latest/filters.html>`_
+following the naming syntax described in `speclite.filters.load_filters`, or
+create their own named `speclite.filters.FilterResponse`.
+
 .. currentmodule:: skypy.galaxies.spectrum
 .. autosummary::
    :nosignatures:

docs/utils/index.rst

@@ -126,6 +126,14 @@ Photometry (`skypy.utils.photometry`)
 This module contains methods that model spectral energy distributions and
 calculate photometric properties.
 
+SkyPy uses the `speclite <https://speclite.readthedocs.io/>`_ package for
+photometric calculations. Some of the following functions take the names of
+photometric filters as an input parameter. Users can choose from the available
+`Speclite Filters <https://speclite.readthedocs.io/en/latest/filters.html>`_
+following the naming syntax described in `speclite.filters.load_filters`, or
+create their own named `speclite.filters.FilterResponse`.
+
+
 .. currentmodule:: skypy.utils.photometry
 .. autosummary::
    :nosignatures: