Add documentation for features.clearsky to API docs

Also add attribution in liscences file.

docs/api.rst

@@ -97,6 +97,7 @@ Functions for detecting features in the data.
    :toctree: generated/
 
    features.clipping_levels
+   features.clearsky
 
 .. rubric:: References
 

docs/licenses.rst

@@ -31,7 +31,7 @@ the terms of the MIT License
 
 * The implementation of the QCRad algorithm in
   :py:mod:`pvanalytics.quality.irradiance`
-  
+
 * The clearsky limits quality check
   :py:func:`pvanalytics.quality.irradiance.clearsky_limits`
 
@@ -49,3 +49,6 @@ the terms of the MIT License
 
 * Level-based clipping detection in
   :py:func:`pvanalytics.features.clipping_levels`.
+
+* Clear sky detection in
+  :py:func:`pvanalytics.features.clearsky`

pvanalytics/features.py

@@ -94,6 +94,38 @@ def clearsky(ghi, ghi_clearsky):
         Global horizontal irradiance in :math:`W/m^2` under clearsky
         conditions.
 
+    Returns
+    -------
+    Series
+        True when clear sky conditions are indicated.
+
+    Raises
+    ------
+    ValueError
+        if the time intervals are greater than 15 minutes.
+
+    Notes
+    -----
+    Clear-sky conditions are inferred when each of six criteria are
+    met; see :py:func:`pvlib.clearsky.detect_clearsky` for references
+    and details. Threshold values for each criterion were originally
+    developed for ten minute windows containing one-minute data
+    [1]_. As indicated in [2]_, the algorithm also works for longer
+    windows and data at different intervals if threshold criteria are
+    roughly scaled to the window length. Here, the threshold values
+    are based on [1] with the scaling indicated in [2].
+
+    References
+    ----------
+    .. [1] Reno, M.J. and C.W. Hansen, "Identification of periods of
+       clear sky irradiance in time series of GHI measurements"
+       Renewable Energy, v90, p. 520-531, 2016.
+
+    .. [2] B. H. Ellis, M. Deceglie and A. Jain, "Automatic Detection
+       of Clear-Sky Periods From Irradiance Data," in IEEE Journal of
+       Photovoltaics, vol. 9, no. 4, pp. 998-1005, July 2019. doi:
+       10.1109/JPHOTOV.2019.2914444
+
     """
     delta = ghi.index.to_series().diff()
     delta_minutes = delta[1] / np.timedelta64(1, '60s')