Update SDL to 2.0.18

libs/SDL2/docs/README-hg.md

@@ -1,22 +1,4 @@
-Mercurial
-=========
-
-The latest development version of SDL is available via Mercurial.
-Mercurial allows you to get up-to-the-minute fixes and enhancements;
-as a developer works on a source tree, you can use "hg" to mirror that
-source tree instead of waiting for an official release. Please look
-at the Mercurial website ( https://www.mercurial-scm.org/ ) for more
-information on using hg, where you can also download software for
-Mac OS X, Windows, and Unix systems.
-
-    hg clone http://hg.libsdl.org/SDL
-
-If you are building SDL via configure, you will need to run autogen.sh
-before running configure.
-
-There is a web interface to the subversion repository at:
-	http://hg.libsdl.org/SDL/
-
-There is an RSS feed available at that URL, for those that want to
-track commits in real time.
-
+We are no longer hosted in Mercurial. Please see README-git.md for details.
+
+Thanks!
+