Add details in cap mgmt

docs/en/developers/cap-center.md

@@ -1,10 +1,12 @@
 # Managing Capabilities
 
-Developers can install more capabilities (workload types and traits) from any GitHub repo that contains OAM definition objects (i.e. capability center).
+In KubeVela, developers can install more capabilities (i.e. new workload types and traits) from any GitHub repo that contains OAM definition files. We call these GitHub repos as _Capability Centers_. 
 
-## Add Capability Center
+KubeVela is able to discover OAM definition files in this repo automatically and sync them to your own KubeVela platform.
 
-Add and sync a remote center:
+## Add a capability center
+
+Add and sync a capability center in KubeVela:
 
 ```bash
 $ vela cap center config my-center https://github.com/oam-dev/catalog/tree/master/registry
@@ -16,7 +18,11 @@ successfully sync 1/1 from my-center remote center
 sync finished
 ```
 
-## List Capability Centers
+Now, this capability center `my-center` is ready to use.
+
+## List capability centers
+
+You are allowed to add more capability centers and list them.
 
 ```bash
 $ vela cap center ls
@@ -24,13 +30,17 @@ NAME     	ADDRESS
 my-center	https://github.com/oam-dev/catalog/tree/master/registry
 ```
 
-## [Optional] Remove Cap Center
+## [Optional] Remove a capability center
+
+Or, remove one.
 
 ```bash
 $ vela cap center remove my-center
 ```
 
-## List Capabilities
+## List all available capabilities in capability center
+
+Or, list all available capabilities in certain center.
 
 ```bash
 $ vela cap ls my-center
@@ -38,7 +48,13 @@ NAME     	CENTER   	TYPE 	DEFINITION                  	STATUS     	APPLIES-TO
 kubewatch	my-center	trait	kubewatches.labs.bitnami.com	uninstalled	[]
 ```
 
-## Install Capability
+## Install a capability from capability center
+
+Now let's try to install the new trait named `kubewatch` from `my-center` to your own KubeVela platform.
+
+> [KubeWatch](https://github.com/bitnami-labs/kubewatch) is a Kubernetes plugin that watches events and publishes notifications to Slack channel etc. We can use it as a trait to watch important changes of your app and notify the platform administrators via Slack.
+
+Install `kubewatch` trait from `my-center`.
 
 ```bash
 $ vela cap install my-center/kubewatch
@@ -51,7 +67,10 @@ Successfully installed chart (kubewatch) with release name (kubewatch)
 Successfully installed capability kubewatch from my-center
 ```
 
-Check traits installed:
+## Use the newly installed capability
+
+Let's check the `kubewatch` trait appears in your platform firstly:
+
 ```bash
 $ vela traits
 Synchronizing capabilities from cluster⌛ ...
@@ -61,9 +80,50 @@ kubewatch 	trait   	Add a watch for resource
 ...
 ```
 
-## Uninstall Capability
+Great! Now let's deploy an app via Appfile.
 
-> Note: make sure no apps are using the capability before uninstalling.
+
+```bash
+$ cat << EOF > vela.yaml
+name: testapp
+services:
+  testsvc:
+    type: webservice
+    image: crccheck/hello-world
+    port: 8000
+    route:
+      domain: testsvc.example.com
+EOF
+```
+
+```bash
+$ vela up
+```
+
+Then let's add `kubewatch` as a trait in this Appfile.
+
+```bash
+$ cat << EOF >> vela.yaml
+    kubewatch:
+      webhook: https://hooks.slack.com/<your-token>
+EOF
+```
+
+> The `https://hooks.slack.com/<your-token>` is the Slack channel that your platform administrators are keeping an eye on.
+
+Update the deployment:
+
+```
+$ vela up
+```
+
+Now, your platform administrators should receive notifications whenever important changes happen to your app. For example, a fresh new deployment.
+
+![Image of Kubewatch](../../resources/kubewatch-notif.jpg)
+
+## Uninstall a capability
+
+> NOTE: make sure no apps are using the capability before uninstalling.
 
 ```bash
 $ vela cap uninstall my-center/kubewatch