Add changelog for v0.3.1 (#263)

Add changelog for v0.3.1 and link to the new version on README.md

Author: santisoler

README.rst

@@ -141,6 +141,7 @@ Documentation for other versions
 * `Development <http://www.fatiando.org/harmonica/dev>`__ (reflects the *master* branch on
   Github)
 * `Latest release <http://www.fatiando.org/harmonica/latest>`__
+* `v0.3.1 <http://www.fatiando.org/harmonica/v0.3.1>`__
 * `v0.3.0 <http://www.fatiando.org/harmonica/v0.3.0>`__
 * `v0.2.1 <http://www.fatiando.org/harmonica/v0.2.1>`__
 * `v0.2.0 <http://www.fatiando.org/harmonica/v0.2.0>`__

doc/changes.rst

@@ -3,6 +3,26 @@
 Changelog
 =========
 
+Version 0.3.1
+-------------
+
+*Released on: 2021/10/20*
+
+.. image:: https://zenodo.org/badge/DOI/10.5281/zenodo.5585665.svg
+   :alt: Digital Object Identifier for the Zenodo archive
+   :target: https://doi.org/10.5281/zenodo.5585665
+
+Bug fix:
+
+- Package ``requirements.txt`` and update the dependencies list: remove
+  ``scipy`` and add ``scikit-learn``. Exclude ``license_notice.py`` and
+  ``.flake8`` from the ``MANIFEST.in`` (`#261 <https://github.com/fatiando/harmonica/pull/261>`__)
+
+This release contains contributions from:
+
+- Santiago Soler
+
+
 Version 0.3.0
 -------------