Update octant-plugin installation guide

Since we have included antrea-octant-plugin binary
into release assets and have a separate plugin
for traceflow for now, update octant-plugin installation
guide to address the changes.

TODO: Update the doc after merge antrea-traceflow-plugin
into antrea-octant-plugin.

Related: antrea-io#901

docs/octant-plugin-installation.md

@@ -91,18 +91,22 @@ You can follow the steps listed below to install octant and antrea-octant-plugin
     export KUBECONFIG=/etc/kubernetes/admin.conf
     ```
 
-3. Build antrea-octant-plugin.
+3. Get corresponding antrea-octant-plugin binary from release/assets based on your environment.
 
+    For example, you can get antrea-octant-plugin-linux-x86_64 if it meets your operating system  and architecture.
+    Then, rename the binary to antrea-octant-plugin so that you do not need to change any command for step4.
     ```
-    cd plugins/octant
-    make antrea-octant-plugin
+    wget https://github.com/vmware-tanzu/antrea/releases/download/v0.8.0/antrea-octant-plugin-linux-x86_64
+    mv antrea-octant-plugin-linux-x86_64 antrea-octant-plugin
     ```
 
-4. Move antrea-octant-plugin to OCTANT_PLUGIN_PATH.
+4. Move binary antrea-octant-plugin to OCTANT_PLUGIN_PATH and make sure it is executable.
 
     ```
     # If you did not change OCTANT_PLUGIN_PATH, the default folder should be $HOME/.config/octant/plugins.
     mv antrea/plugins/octant/bin/antrea-octant-plugin $HOME/.config/octant/plugins/
+    # Make sure antrea-octant-plugin is executable, otherwise Octant can not find it.
+    chmod a+x antrea-octant-plugin
     ```
 
 5. Start Octant as a background process with UI related environment variables.
@@ -113,3 +117,17 @@ You can follow the steps listed below to install octant and antrea-octant-plugin
     ```
 
 Now, you are supposed to see Octant is running together with antrea-octant-plugin via URL http://(IP or $HOSTNAME):80.
+
+Note:
+1. For Traceflow UI, it is a separate octant plugin called antrea-traceflow-plugin in Antrea release v0.8.0, but
+we plan to merge it into antrea-octant-plugin soon. You can already view the alpha version of traceflow UI if you chooose
+to deploy Antrea UI as a Pod, since we have already included this plugin into image antrea/octant-antrea-ubuntu.
+2. If you deploy Antrea UI as a process, you can not see Traceflow UI for now if you follow the five steps listed above,
+since we have not merged this function into antrea-octant-plugin in Antrea v0.8.0 release assets yet. But you can still
+compile the binary by yourself via command below for step 3 and the remaining steps are almost the same as antrea-octant-plugin.
+
+    ```
+    # You will find the compliled binary under folder antrea/plugins/octant/bin.
+    cd plugins/octant
+    make antrea-traceflow-plugin
+    ```