doc: cover cnpg plugin upgrade with krew (cloudnative-pg#2012)

Closes cloudnative-pg#2011

Signed-off-by: Gabriele Bartolini <gabriele.bartolini@enterprisedb.com>

Author: gbartolini

docs/src/cnpg-plugin.md

@@ -88,6 +88,14 @@ run:
 kubectl krew install cnpg
 ```
 
+When a new version of the plugin is released, you can update the existing
+installation with:
+
+```sh
+kubectl krew update
+kubectl krew upgrade cnpg
+```
+
 ### Supported Architectures
 
 CloudNativePG Plugin is currently built for the following