Update contributing guidelines (#1752)

CONTRIBUTING.rst

@@ -114,6 +114,7 @@ initialize the ``setUpClass`` properly:
     def setUpClass(cls):
         super().setUpClass(Argentina)
 
+
 Build sphinx documentation
 --------------------------
 
@@ -127,3 +128,13 @@ with reStructuredText for Sphinx you can read a primer
 `here`__.
 
 __ https://www.sphinx-doc.org/en/master/usage/restructuredtext/basics.html
+
+
+Contributors
+------------
+
+.. __: https://github.com/vacanza/python-holidays/blob/dev/AUTHORS
+
+In order to keep the list of contributors up to date we encourage you add your
+name (in alphabetical order) to the `AUTHORS`__ file if it's not there yet.
+Thanks for your contribution!