Sync SDL3 wiki -> header

docs/README-git.md

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+git
+=========
+
+The latest development version of SDL is available via git.
+Git allows you to get up-to-the-minute fixes and enhancements;
+as a developer works on a source tree, you can use "git" to mirror that
+source tree instead of waiting for an official release. Please look
+at the Git website ( https://git-scm.com/ ) for more
+information on using git, where you can also download software for
+macOS, Windows, and Unix systems.
+
+    git clone https://github.com/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 Git repository at:
+	http://github.com/libsdl-org/SDL/
+

docs/README-hg.md

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+Mercurial
+======
+
+We are no longer hosted in Mercurial. Please see README-git.md for details.
+
+Thanks!
+