Add reference for APP method of gamma distribution

docs/references.bib

@@ -2103,3 +2103,18 @@ @article{Lafferty2023
    url = {https://www.nature.com/articles/s41612-023-00486-0},
    year = {2023},
 }
+
+@article{thom_1958,
+  title = {A Note on the Gamma Distribution},
+  author = {Thom, H. C. S.},
+  year = {1958},
+  journal = {Monthly Weather Review},
+  volume = {86},
+  number = {4},
+  pages = {117--122},
+  publisher = {{American Meteorological Society}},
+  issn = {1520-0493, 0027-0644},
+  doi = {10.1175/1520-0493(1958)086<0117:ANOTGD>2.0.CO;2},
+  abstract = {Abstract The general properties of the gamma distribution, which has several applications in meteorology, are discussed. A short review of the general properties of good statistical estimators is given. This is applied to the gamma distribution to show that the maximum likelihood estimators are jointly sufficient. A new, simple approximation of the likelihood solutions is given, and the efficiency of the fitting procedure is computed.},
+  chapter = {Monthly Weather Review},
+}

xclim/indices/stats.py

@@ -476,7 +476,7 @@ def _fit_start(x, dist: str, **fitkwargs: Any) -> tuple[tuple, dict]:
 
     References
     ----------
-    :cite:cts:`coles_introduction_2001,cohen_parameter_2019`
+    :cite:cts:`coles_introduction_2001,cohen_parameter_2019, thom_1958`
 
     """
     x = np.asarray(x)