Fix README hyperlinks under Credits

Change README hyperlinks from Markdown style to RST style. Make URLs
HTTPS. Correct some grammar.

README.rst

@@ -50,6 +50,6 @@ Contributing
 
 Credits
 -------
- - Icons made by [Freepik](http://www.freepik.com) from
-   [www.flaticon.com](https://www.flaticon.com/) is licensed by
-   [CC 3.0 BY](http://creativecommons.org/licenses/by/3.0/)
+ - Icons made by `Freepik <https://www.freepik.com>`_ from
+   `Flaticon <https://www.flaticon.com/>`_ are licensed under
+   `CC 3.0 BY <https://creativecommons.org/licenses/by/3.0/>`_.