Add SkyWalking CLI download links (#4038)

docs/README.md

@@ -43,8 +43,10 @@ If you have been familiar with SkyWalking, you could use this catalog to find th
     * [Query in OAP](en/protocols/README.md#query-protocol). A set of query protocol provided, based on the Observability Analysis Language metrics definition.
   * UI
     * [Overview](en/concepts-and-designs/ui-overview.md).  A simple brief about SkyWalking UI.
+  * CLI (Command Line Interface)
+    * SkyWalking CLI provides a command line interface to interact with SkyWalking backend (via GraphQL), for more information, [click here](https://github.com/apache/skywalking-cli).
 * [Setup](en/setup/README.md).
-  * Backend, UI and Java agent are Apache official release, you could find them at [Apache SkyWalking DOWNLOAD page](http://skywalking.apache.org/downloads/).
+  * Backend, UI, Java agent, and CLI are Apache official release, you could find them at [Apache SkyWalking DOWNLOAD page](http://skywalking.apache.org/downloads/).
   * Language agents in Service
     * [Java agent](en/setup/service-agent/java-agent/README.md). Introduce how to install java agent to your service, without change any codes.
       * [Supported middleware, framework and library](en/setup/service-agent/java-agent/Supported-list.md).
@@ -58,7 +60,7 @@ If you have been familiar with SkyWalking, you could use this catalog to find th
   * Service Mesh
     * [SkyWalking on Istio](en/setup/istio/README.md). Introduce how to use Istio Mixer bypass Adapter to work with SkyWalking.
     * Use [ALS(access log service)](https://www.envoyproxy.io/docs/envoy/latest/api-v2/service/accesslog/v2/als.proto) to observe service mesh, without Mixer. Follow [document](en/setup/envoy/als_setting.md) to open it.
-  * [Backend and UI setup document](en/setup/backend/backend-ui-setup.md).
+  * [Backend, UI and CLI setup document](en/setup/backend/backend-ui-setup.md).
     * [Backend setup document](en/setup/backend/backend-setup.md).
       * [Overriding settings](en/setup/backend/backend-setting-override.md) in application.yml is supported。
       * [IP and port setting](en/setup/backend/backend-ip-port.md). Introduce how IP and port set and be used.
@@ -77,6 +79,7 @@ If you have been familiar with SkyWalking, you could use this catalog to find th
       * [Dynamic Configuration](en/setup/backend/dynamic-config.md). Make configuration of OAP changed dynamic, from remote service or 3rd party configuration management system.
       * [Uninstrumented Gateways](en/setup/backend/uninstrumented-gateways.md). Configure gateways/proxies that are not supported by SkyWalking agent plugins, to reflect the delegation in topology graph.
     * [UI setup document](en/setup/backend/ui-setup.md).
+    * [CLI setup document](https://github.com/apache/skywalking-cli).
 * [Contributing Guides](en/guides/README.md). Guides are for PMC member, committer or new contributor. At here, you can know how to contribute from beginning.
   * [Contact us](en/guides/README.md#contact-us). Guide users about how to contact the official committer team or communicate with the project community.
   * [Process to become official Apache SkyWalking Committer](en/guides/asf/committer.md). How to become an official committer or PMC member.

docs/en/concepts-and-designs/README.md

@@ -21,3 +21,5 @@ SkyWalking already support.
   - [Query in OAP](../protocols/README.md#query-protocol). A set of query protocol provided, based on the Observability Analysis Language metrics definition. 
 - UI
   - [Overview](ui-overview.md). A simple brief about SkyWalking UI.
+- CLI
+  - [SkyWalking CLI](https://github.com/apache/skywalking-cli). A command line interface for SkyWalking.

docs/en/setup/backend/backend-ui-setup.md

-# Backend and UI
+# Backend, UI, and CLI
 
 SkyWalking backend distribution package includes following parts
 1. **bin/cmd scripts**, in `/bin` folder. Include startup linux shell and Windows cmd scripts for Backend
@@ -34,7 +34,7 @@ Before that, you should know how agent/probe, backend, UI communicate with each
 data to backend. Also, jetty service supported in JSON format. 
 - UI uses GraphQL(HTTP) query to access backend also in Jetty service(`core/default/rest*` in `application.yml`).
 
-Now, let's continue with the backend and UI setting documents.
+Now, let's continue with the backend, UI and CLI setting documents.
 - [Backend setup document](backend-setup.md)
 - [UI setup document](ui-setup.md)
-
+- [CLI set up document](https://github.com/apache/skywalking-cli)