Add the documentation for auto-deployment (#253)

Author: Noah Lee

docs/references/deploy.yml.md

@@ -17,8 +17,8 @@ Field                    |Type                     |Required  |Description
 `required_contexts`      |*[]string*               |`false`   |This field allows you to specify a subset of contexts that must be success. 
 `payload`                |*object* or *string*     |`false`   |This field is JSON payload with extra information about the deployment. 
 `production_environment` |*boolean*                |`false`   |This field specifies whether this runtime environment is production or not.
-`deployable_ref`         |*string*                 |`false`   |This field specifies which the ref(branch, SHA, tag) is deployable or not. It supports the regular expression, [re2](https://github.com/google/re2/wiki/Syntax) by Google, to match the ref. 
-`auto_deploy_on`         |*string*                 |`false`   |This field controls auto-deployment behaviour given a ref(branch, SHA, tag). It supports the regular expression, [re2](https://github.com/google/re2/wiki/Syntax) by Google, to match the ref. 
+`deployable_ref`         |*string*                 |`false`   |This field specifies which the ref(branch, SHA, tag) is deployable or not. It supports the regular expression ([re2]((https://github.com/google/re2/wiki/Syntax))). 
+`auto_deploy_on`         |*string*                 |`false`   |This field controls auto-deployment behaviour given a ref(branch, SHA, tag). If any new push events are detected on this event, the deployment will be triggered. It supports the regular expression ([re2](https://github.com/google/re2/wiki/Syntax)). E.g. `refs/heads/main` or `refs/tags/v.*`
 `review`                 |*[Review](#review)*      |`false`   |This field configures review.
 
 ## Review

docs/tasks/usecases.md

@@ -37,6 +37,20 @@ envs:
     production_environment: true
 ```
 
+## Auto deployment
+
+If you want to enable the auto-deployment when the pull request is merged into the main branch, you should configure the `auto_deploy_on` field like the below.
+
+```yaml
+envs:
+  - name: production
+    auto_merge: true
+    required_contexts: []
+    auto_deploy_on: refs/heads/main
+    deployable_ref: main
+    production_environment: true
+```
+
 ## Questions?
 
 We are always happy to help with questions you might have. You can post questions or comments to our [community](https://github.com/gitploy-io/gitploy/discussions).