update ambiguous licensing

see https://github.com/CelestiaProject/Celestia/pull/831 for more info

Thanks to Tom Callaway <spot@fedoraproject.org> to his efforts!

README

@@ -126,6 +126,72 @@ https://www.deviantart.com/celestiaofficial/gallery/68412929/Release-Textures
   The map of Enceladus is derived from the December 2008 CICLOPS map:
   http://ciclops.org/view/5447/Map_of_Enceladus_December_2008
 
+Note on content from JPL
+------------------------
+Some content in Celestia, including texture maps and models, comes from JPL
+websites. That content is subject to the JPL Image Use Policy, a copy of which
+appears below (as captured on 2020-July-13). The latest version of this policy
+can be found at https://www.jpl.nasa.gov/imagepolicy/. Also refer to
+https://space.jpl.nasa.gov/faq.html for clarity on how textures/models are
+covered.
+
+# JPL Image Use Policy
+# Unless otherwise noted, images and video on JPL public web sites (public
+# sites ending with a jpl.nasa.gov address) may be used for any purpose
+# without prior permission, subject to the special cases noted below.
+# Publishers who wish to have authorization may print this page and
+# retain it for their records; JPL does not issue image permissions on an
+# image by image basis.
+#
+# By electing to download the material from this web site the user agrees:
+# 1. that Caltech makes no representations or warranties with respect to
+# ownership of copyrights in the images, and does not represent others who
+# may claim to be authors or owners of copyright of any of the images, and
+# makes no warranties as to the quality of the images. Caltech shall not be
+# responsible for any loss or expenses resulting from the use of the images,
+# and you release and hold Caltech harmless from all liability arising from
+# such use.
+#
+# 2. to use a credit line in connection with images. Unless otherwise noted
+# in the caption information for an image, the credit line should be
+# "Courtesy NASA/JPL-Caltech."
+#
+# 3. that the endorsement of any product or service by Caltech, JPL or NASA
+# must not be claimed or implied.
+#
+# Special Cases:
+# * Prior written approval must be obtained to use the NASA insignia logo
+# (the blue "meatball" insignia), the NASA logotype (the red "worm" logo)
+# and the NASA seal. These images may not be used by persons who are not
+# NASA employees or on products (including Web pages) that are not NASA
+# sponsored. In addition, no image may be used to explicitly or implicitly
+# suggest endorsement by NASA, JPL or Caltech of commercial goods or
+# services. Requests to use NASA logos may be directed to Bert Ulrich, Public
+# Services Division, NASA Headquarters, Code POS, Washington, DC 20546,
+# telephone (202) 358-1713, fax (202) 358-4331, email bert.ulrich@hq.nasa.gov.
+#
+# * Prior written approval must be obtained to use the JPL logo (stylized JPL
+# letters in red or other colors). Requests to use the JPL logo may be
+# directed to the Institutional Communications Office, email
+# instcomm@jpl.nasa.gov.
+#
+# * If an image includes an identifiable person, using the image for
+# commercial purposes may infringe that person's right of privacy or publicity,
+# and permission should be obtained from the person. NASA and JPL generally do
+# not permit likenesses of current employees to appear on commercial products.
+# For more information, consult the NASA and JPL points of contact listed above.
+#
+# * JPL/Caltech contractors and vendors who wish to use JPL images in
+# advertising or public relation materials should direct requests to the
+# Institutional Communications Office, email instcomm@jpl.nasa.gov.
+#
+# * Some image and video materials on JPL public web sites are owned by
+# organizations other than JPL or NASA. These owners have agreed to make their
+# images and video available for journalistic, educational and personal uses,
+# but restrictions are placed on commercial uses. To obtain permission for
+# commercial use, contact the copyright owner listed in each image caption.
+# Ownership of images and video by parties other than JPL and NASA is noted in
+# the caption material with each image.
 
 3D Models
 ---------
@@ -135,6 +201,9 @@ https://www.deviantart.com/celestiaofficial/gallery/68412929/Release-Textures
 
 # Models of Mars Global Surveyor and Mars Odyssey were created by Shrox:
     http://www.shrox.com/
+  They are included under the Creative Commons Attribution 4.0 International
+  License (CC BY 4.0):
+    https://creativecommons.org/licenses/by/4.0/
 
 # The Cassini and Huygens models are by Jack Higgins:
     http://homepage.eircom.net/~jackcelestia/
@@ -150,7 +219,10 @@ https://www.deviantart.com/celestiaofficial/gallery/68412929/Release-Textures
 
 # 3D asteroid models of Toutatis, Kleopatra, Geographos, 1998 KY26, Bacchus,
   Castalia and Golevka are courtesy of Scott Hudson, Washington State
-  University.
+  University. Originally found here:
+    http://users.tricity.wsu.edu/~hudson/Research/Asteroids/index.htm
+  On August 20, 2020, Scott Hudson waived the commercial use restrictions on
+  these models, permitting unrestricted use of them.
 
 # 3D models of Amalthea, Janus, Epimetheus, Prometheus, Pandora,
   Hyperion, Larissa, Proteus, Vesta, Ida, Gaspra and Halley are derived from