Missing corresponding asset file for fullNuclideChart image

[Luisav1]

From #112.

Is there a file in assets/ for every resource file in sound/ and images/? Note that there is not necessarily a
1:1 correspondence between asset and resource files; for example, several related images may be in the same .ai file.
Check license.json for possible documentation of why some resources might not have a corresponding asset file

[Luisav1]

In meeting with @zepumph, decided we need to add a note to the license on why an asset file is not needed and the link to where it came from (it was just a screenshot from the 3D nuclide chart, not modified).

[Luisav1]

Added documentation for why an asset file is not needed in commit above. Closing.