Mention use of SDL and SDL_image libraries in the READMEs and that they fall

under the LGPL 2.1 license which has been added.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@9949 c06c8d41-db1a-0410-9941-cceddc491573

Created by  dploog  on June 10, 2009
3OR2FJJNMAOBE7EUQAPOIQVBSNDKHPEQWFACS2AS73CDPQZDFYEAC
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