. : added Procfile.learner

Added Procfile.learner to easily start cluster with learner
Modified README.md to highlight its usage
Update review comments

Procfile

@@ -3,3 +3,5 @@ etcd1: bin/etcd --name infra1 --listen-client-urls http://127.0.0.1:2379 --adver
 etcd2: bin/etcd --name infra2 --listen-client-urls http://127.0.0.1:22379 --advertise-client-urls http://127.0.0.1:22379 --listen-peer-urls http://127.0.0.1:22380 --initial-advertise-peer-urls http://127.0.0.1:22380 --initial-cluster-token etcd-cluster-1 --initial-cluster 'infra1=http://127.0.0.1:12380,infra2=http://127.0.0.1:22380,infra3=http://127.0.0.1:32380' --initial-cluster-state new --enable-pprof --logger=zap --log-outputs=stderr
 etcd3: bin/etcd --name infra3 --listen-client-urls http://127.0.0.1:32379 --advertise-client-urls http://127.0.0.1:32379 --listen-peer-urls http://127.0.0.1:32380 --initial-advertise-peer-urls http://127.0.0.1:32380 --initial-cluster-token etcd-cluster-1 --initial-cluster 'infra1=http://127.0.0.1:12380,infra2=http://127.0.0.1:22380,infra3=http://127.0.0.1:32380' --initial-cluster-state new --enable-pprof --logger=zap --log-outputs=stderr
 #proxy: bin/etcd grpc-proxy start --endpoints=127.0.0.1:2379,127.0.0.1:22379,127.0.0.1:32379 --listen-addr=127.0.0.1:23790 --advertise-client-url=127.0.0.1:23790 --enable-pprof
+
+# A learner node can be started using Procfile.learner

Procfile.learner

@@ -0,0 +1,11 @@
+# Use goreman to run `go get github.com/mattn/goreman`
+
+# 1. Start the cluster using Procfile
+# 2. Add learner node to the cluster
+#   % etcdctl member add infra4 --peer-urls="http://127.0.0.1:42380" --learner=true
+
+# 3. Start learner node with goreman
+etcd4: bin/etcd --name infra4 --listen-client-urls http://127.0.0.1:42379 --advertise-client-urls http://127.0.0.1:42379 --listen-peer-urls http://127.0.0.1:42380 --initial-advertise-peer-urls http://127.0.0.1:42380 --initial-cluster-token etcd-cluster-1 --initial-cluster 'infra4=http://127.0.0.1:42380,infra1=http://127.0.0.1:12380,infra2=http://127.0.0.1:22380,infra3=http://127.0.0.1:32380' --initial-cluster-state existing --enable-pprof --logger=zap --log-outputs=stderr
+
+# 4. The learner node can be promoted to voting member by the command
+#   % etcdctl member promote <memberid>

README.md

@@ -135,6 +135,12 @@ This will bring up 3 etcd members `infra1`, `infra2` and `infra3` and etcd `grpc
 
 Every cluster member and proxy accepts key value reads and key value writes.
 
+Follow the steps in [Procfile.learner](./Procfile.learner) to add a learner node to the cluster. Start the learner node with:
+
+```bash
+goreman -f ./Procfile.learner start
+```
+
 ### Next steps
 
 Now it's time to dig into the full etcd API and other guides.