Update readme of 6.1.0

6/6.1/oap/README.md

+# Apache SkyWalking OAP Server Docker Files
+
+<img src="http://skywalking.apache.org/assets/logo.svg" alt="Sky Walking logo" height="90px" align="right" />
+
+**SkyWalking**: an APM(application performance monitor) system, especially designed for 
+microservices, cloud native and container-based (Docker, Kubernetes, Mesos) architectures.
+
+# How to use this image
+
+## Start a `standlone` container with `H2` storage
+
+```
+$ docker run --name oap --restart always -d apache/skywalking-oap-server:6.1.0
+```
+
+## Start a `standlone` container with `elasticsearch` storage whose address is `elasticsearch:9200`
+
+```
+$ docker run --name oap --restart always -d -e SW_STORAGE=elasticsearch -e SW_STORAGE_ES_CLUSTER_NODES=elasticsearch:9200 apache/skywalking-oap-server:6.1.0
+```
+
+# Configuration
+
+We could set up environment variables to configure this image. Most of them are defined in [backend-setup](https://github.com/apache/skywalking/blob/v6.1.0/docs/en/setup/backend/backend-setup.md), but Apache SkyWalking Docker Image adds extra environment variables to help the user to compose it properly. The details are located in [docker-entrypoint.sh](docker-entrypoint.sh)
+
+## SW_CLUSTER
+Default value is `standalone`, avaliable values are:
+
+ - `standlone`, If set this value, all enviroment variables of `cluster.standlone` in `application.yml` are avaliable.
+ - `zookeeper`, If set this value, all enviroment variables of `cluster.zookeeper` in `application.yml` are avaliable.
+ - `kubernetes`, If set this value, all enviroment variables of `cluster.kubernetes` in `application.yml` are avaliable.
+ - `consul`, If set this value, all enviroment variables of `cluster.consul` in `application.yml` are avaliable.
+ 
+## SW_STORAGE
+Default value is `H2`, avaliable values are:
+
+ - `H2`, If set this value, all enviroment variables of `storage.h2` in `application.yml` are avaliable.
+ - `elasticsearch`, If set this value, all enviroment variables of `stroage.elasticsearch` in `application.yml` are avaliable.
+ - `mysql`, If set this value, all enviroment variables of `stroage.mysql` in `application.yml` are avaliable.
+ 
+## XXX_ENABLED
+
+There are also some other configuration switchers to extend default configuration, they also have a suffix `_ENABLED`.
+
+ - `SW_RECEIVER_ZIPKIN_ENABLED` turns on/off zipkin receiver
+ - `SW_RECEIVER_JAEGER_ENABLED` turns on/off jaeger receiver
+ - `SW_EXPORTER_ENABLED`  turns on/off exporter
+
+# License
+[Apache 2.0 License.](/LICENSE)