Fix doc

bootstrap-pull-request/README.md

@@ -1,6 +1,7 @@
 # bootstrap-pull-request [![bootstrap-pull-request](https://github.com/quipper/monorepo-deploy-actions/actions/workflows/bootstrap-pull-request.yaml/badge.svg)](https://github.com/quipper/monorepo-deploy-actions/actions/workflows/bootstrap-pull-request.yaml)
 
 This is an action to bootstrap the pull request namespace.
+It should be called when a pull request is created or updated.
 
 ## Getting Started
 
@@ -35,7 +36,7 @@ This action creates a namespace branch into the destination repository.
 ns/${source-repository}/${overlay}/${namespace-prefix}${pull-request-number}
 ```
 
-It assumes the below directory structure of namespace branch.
+It creates the following directory structure.
 
 ```
 .
@@ -47,6 +48,12 @@ It assumes the below directory structure of namespace branch.
         └── generated.yaml
 ```
 
+It assumes that the below name of prebuilt branch exists in the destination repository.
+
+```
+prebuilt/${source-repository}/${overlay}
+```
+
 It bootstraps the namespace branch by the following steps:
 
 - Copy the services from prebuilt branch.
@@ -71,6 +78,10 @@ the namespace branch will be the below structure.
         └── generated.yaml
 ```
 
+All placeholders will be replaced during copying the service manifests.
+For example, if `NAMESPACE=pr-123` is given by `substitute-variables` input,
+this action will replace `${NAMESPACE}` with `pr-123`. 
+
 If a service was pushed by other action such as `git-push-service`,
 this action does not overwrite it.
 
@@ -98,6 +109,10 @@ This action copies the namespace manifest to path `/applications/namespace.yaml`
     └── namespace.yaml
 ```
 
+All placeholders will be replaced during copying the namespace manifest.
+For example, if `NAMESPACE=pr-123` is given by `substitute-variables` input,
+this action will replace `${NAMESPACE}` with `pr-123`. 
+
 ## Specification
 
 See [action.yaml](action.yaml).