docs/benchmark: add benchmark result links in README

Documentation/benchmarks/README.md

@@ -2,4 +2,6 @@
 
 etcd benchmarks will be published regularly and tracked for each release below:
 
-- [etcd v2.1.0](etcd-2-1-0-benchmarks.md)
+- [etcd v2.1.0-alpha](./etcd-2-1-0-alpha-benchmarks.md)
+- [etcd v2.2.0-rc](./etcd-2-2-0-rc-benchmarks.md)
+- [etcd v3 demo](./etcd-3-demo-benchmarks.md)