docs: clarify installation by Kustomize (apache#558)

install.md

@@ -40,6 +40,10 @@ kubectl kustomize "github.com/apache/apisix-ingress-controller/samples/deploy?re
 
 Parameters are hardcoded so if the default values are not good for you, just tweak them manually.
 
+To tweak parameters, first you need to modify config files in _samples/deploy_ directory. There are many ways to acheive this. For example, you may insert a `sed` command after `kubectl kustomize`, that is, `kubectl kustomize "github.com/apache/apisix-ingress-controller/samples/deploy?ref=master" | sed "s@to-be-modified@after-modified@g" | kubectl apply -f -`. Another way is to use a local copy of _samples/deploy_ directory or a copy from your repo.
+
+Then you need to know which parameter need to be tweaked. If APISIX access token or the address of APISIX Admin API is changed, you need to modify `apisix.admin_key` or `apisix.base_url` respectively in field `.data.config.yaml` in file _samples/deploy/configmap/apisix-ingress-cm.yaml_. Another example is to install apisix-ingress-controller with different version, in which case you need to configure `.spec.template.spec.containers.[image]` to a desired version in file _samples/deploy/deployment/ingress-controller.yaml_.
+
 ## Verify Installation
 
 There are a lot of use examples (See [samples](docs/en/latest/practices/index.md) for more details), try to follow the operations there to verify the installation.