Merge pull request #3394 from yichengq/bench-2.2

adjust file and README in docs/benchmark

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)

Documentation/benchmarks/etcd-2-1-0-benchmarks.md → Documentation/benchmarks/etcd-2-1-0-alpha-benchmarks.md

@@ -6,7 +6,7 @@ GCE n1-highcpu-2 machine type
 - 1x dedicated slow disk for the OS
 - 1.8 GB memory
 - 2x CPUs
-- etcd version 2.1.0
+- etcd version 2.1.0 alpha
 
 ## etcd Cluster