Relicense the logo as CC-BY-SA-4.0 (#2786)

and add the original svg file as well.

Author: douardda

ChangeLog

@@ -22,6 +22,7 @@ Release date: TBA
   This check is emitted when ``pylint`` finds a class variable that conflicts with a slot
   name, which would raise a ``ValueError`` at runtime.
 
+* Relicense logo material under the CC BY-SA 4.0 license.
 
 What's New in Pylint 2.3.0?
 ===========================

README.rst

@@ -39,7 +39,8 @@ from within your code, as well as from an extensive configuration file.
 It is also possible to write your own plugins for adding your own checks or for
 extending pylint in one way or another.
 
-It's a free software distributed under the GNU General Public Licence.
+It's a free software distributed under the GNU General Public Licence unless
+otherwise specified.
 
 Development is hosted on GitHub: https://github.com/PyCQA/pylint/
 
@@ -102,3 +103,15 @@ For more detailed information, check the documentation.
 
 .. _here: http://pylint.pycqa.org/en/latest/user_guide/installation.html
 .. _tox: https://tox.readthedocs.io/en/latest/
+
+License
+-------
+
+pylint is, with a few exceptions listed below, `GPLv2 <COPYING>`_.
+
+The icon files are licensed under the `CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0/>`_ license:
+
+- `doc/logo.png <doc/logo.png>`_
+- `doc/logo.svg <doc/logo.svg>`_
+
+