Replaced README.SVN with Mercurial information, for 1.2 branch.

README.HG

@@ -0,0 +1,23 @@
+
+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 ( http://mercurial.selenic.com/ ) for more
+information on using hg, where you can also download software for
+Mac OS X, Windows, and Unix systems.
+
+  hg clone -u SDL-1.2 http://hg.libsdl.org/SDL
+
+If you are building SDL with an IDE, you will need to copy the file
+include/SDL_config.h.default to include/SDL_config.h before building.
+
+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.
+