i.evapo.time: Add example image to manual (#3281)

* Update i.evapo.time.html

Improved the explaination of the module with a figure and a reference

* Add example of temporal integration (#3280)

Ref: Chemin and Alexandridis 2004 https://ikee.lib.auth.gr/record/270217/files/Chemin-Alexandridis.pdf

---------

Co-authored-by: Edouard Choinière <27212526+echoix@users.noreply.github.com>

imagery/i.evapo.time/i.evapo.time.html

@@ -50,6 +50,21 @@ <h2>NOTES</h2>
 For multi-year calculations, just continue incrementing DOY values above
 366, it will continue working, up to maximum input of 400 satellite images.
 
+<p>
+<img src="i_evapo_time.png" alt="Temporal integration from a weather station"><br>
+<i>This is an example of a temporal integration from a weather station as done by 
+Chemin and Alexandridis (2004)</i>
+<p>
+
+<h2>References</h2>
+
+<p>
+Chemin and Alexandridis, 2004. Spatial Resolution Improvement of Seasonal
+Evapotranspiration for Irrigated Rice, Zhanghe Irrigation District, Hubei Province, China. 
+Asian Journal of Geoinformatics, Vol. 5, No. 1, September 2004
+(<a href="https://ikee.lib.auth.gr/record/270217/files/Chemin-Alexandridis.pdf">PDF</a>)
+
+
 <h2>SEE ALSO</h2>
 
 <em>