feat: inherit field for release template inheritance (#606)

* feat: `inherit` field for release template inheritance

Ref https://github.com/helmfile/helmfile/issues/435#issuecomment-1364749414

Signed-off-by: Yusuke Kuoka <ykuoka@gmail.com>

* Fix wording

Signed-off-by: Yusuke Kuoka <ykuoka@gmail.com>

* Comment on releaseWithInheritedTemplate

Signed-off-by: Yusuke Kuoka <ykuoka@gmail.com>

* Update Release Template doc with the new `inherit` feature

Signed-off-by: Yusuke Kuoka <ykuoka@gmail.com>

* Fix a typo in code comment

Signed-off-by: Yusuke Kuoka <ykuoka@gmail.com>

Signed-off-by: Yusuke Kuoka <ykuoka@gmail.com>

docs/writing-helmfile.md

@@ -85,7 +85,7 @@ It allows you to abstract away the repetitions in releases into a template, whic
 
 ```yaml
 templates:
-  default: &default
+  default:
     chart: stable/{{`{{ .Release.Name }}`}}
     namespace: kube-system
     # This prevents helmfile exiting when it encounters a missing file
@@ -102,10 +102,12 @@ templates:
 releases:
 - name: heapster
   version: 0.3.2
-  <<: *default
+  inherit:
+    template: default
 - name: kubernetes-dashboard
   version: 0.10.0
-  <<: *default
+  inherit:
+    template: default
 ```
 
 Release Templating supports the following parts of release definition:
@@ -144,7 +146,13 @@ Release Templating supports the following parts of release definition:
   # ...
   ```
 
-See the [issue 428](https://github.com/roboll/helmfile/issues/428) for more context on how this is supposed to work.
+Previously, we've been using YAML anchors for release template inheritance.
+It turned out not work well when you wanted to nest templates for complex use cases and/or you want a fine control over which fields to inherit or not.
+Thus we added a new way for inheritance, which uses the `inherit` field we introduced above.
+
+See [issue helmfile/helmfile#435](https://github.com/helmfile/helmfile/issues/435#issuecomment-1362177510) for more context.
+
+You might also find [issue roboll/helmfile#428](https://github.com/roboll/helmfile/issues/428) useful for more context on how we originally designed the relase template and what it's supposed to solve.
 
 ## Layering Release Values