Add a link to the simulator from the top-level README

Hopefully this will help a little with awareness of the existence of the
simulator.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/README.md b/README.md
index 178ab2b..289a14d 100644
--- a/README.md
+++ b/README.md
@@ -25,6 +25,7 @@
 - [Zephyr](docs/readme-zephyr.md)
 - [Mynewt](docs/readme-mynewt.md)
 - [RIOT](docs/readme-riot.md)
+- [Simulator](sim/README.rst)
 
 ## Roadmap