README.rst: add .rst symlink for GitHub.

Signed-off-by: W. Trevor King <wking@tremily.us>

diff --git a/README.rst b/README.rst
new file mode 120000 (symlink)
index 0000000..100b938
--- /dev/null
+++ b/README.rst
@@ -0,0 +1 @@
+README
\ No newline at end of file