Add changelog entry for v0.1.0

[santisoler]

Add doc/changes.rst file containing a changelog.
Add instructions on how to cite Harmonica to documentation.
Add Zenodo badge on README.rst and link to the future v0.1.0 documentation.
Add conda and pip installing instructions in doc/install.rst.

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to the docstring, gallery, and/or tutorials.

[leouieda]

Looks good to me! After #141 is merged this is good to go. Need to update the release date in the changelog as well.