Noting GPLv3 licensed font

[hyandell]

I just wanted to note that the following file is under GPLv3, but is not mentioned on the subsequent license file:

• https://github.com/python-pillow/Pillow/blob/master/Tests/fonts/FreeMono.ttf (run strings on the file and the licensing is contained within)
• https://github.com/python-pillow/Pillow/blob/master/Tests/fonts/LICENSE.txt

As the licenses mentioned in the LICENSE.txt are more in line with Pillow's license, I wanted to flag the FreeMono.ttf license in case it was a surprise and wanted to be mentioned in LICENSE.txt, or removed.

[radarhere]

Just for reference, the font file was added in #209, a continuation of #202.

I've created #5002 to note the license.