Add to reference and add whatsnew entry

Author: echedey-ls

docs/sphinx/source/reference/effects_on_pv_system_output/spectrum.rst

@@ -10,3 +10,4 @@ Spectrum
    spectrum.get_example_spectral_response
    spectrum.get_am15g
    spectrum.calc_spectral_mismatch_field
+   spectrum.martin_ruiz

docs/sphinx/source/whatsnew/v0.9.5.rst

@@ -18,6 +18,9 @@ Enhancements
 ~~~~~~~~~~~~
 * Added the optional `string_factor` parameter to
   :py:func:`pvlib.snow.loss_townsend` (:issue:`1636`, :pull:`1653`)
+* Added :py:func:`pvlib.spectrum.martin_ruiz`, a spectral
+  mismatch correction factor for POA components, dependant in module type,
+  airmass and clearness index (:pull:`1658`)
 
 Bug fixes
 ~~~~~~~~~
@@ -69,3 +72,4 @@ Contributors
 * Anton Driesse (:ghuser:`adriesse`)
 * Adam R. Jensen (:ghuser:`AdamRJensen`)
 * Michael Deceglie (:ghuser:`mdeceglie`)
+* Echedey Luis (:ghuser:`echedey-ls`)