Make README a symlink to README.md

So as not to confuse anyone who has linked to the README file on github in
the past.

diff --git a/README b/README
new file mode 120000 (symlink)
index 0000000..42061c0
--- /dev/null
+++ b/README
@@ -0,0 +1 @@
+README.md
\ No newline at end of file