Add explanations around Docker images

Author: takanabe

Dockerfile

@@ -1,4 +1,4 @@
-FROM takanabe/github-actions-automate-projects:v0.0.1
+FROM takanabe/github-actions-automate-projects:latest
 
 LABEL "com.github.actions.name"="Automate projects"
 LABEL "com.github.actions.description"="GitHub Actions adding GitHub Issues & Pull requests to the specified GitHub Project column automatically ♻️"

README.md

@@ -12,7 +12,6 @@ GitHub Actions adding GitHub Issues & Pull requests to the specified GitHub Proj
 
 GitHub Projects belong to organizations, repositories, and users. This GitHub action currently does not support user-based GitHub Project. For any type of GitHub Projects, you need to change `GITHUB_PROJECT_URL` and `GITHUB_PROJECT_COLUMN_NAME` depending on your GitHub Project URL and column name to which you want to add new cards. Create `.github/workflows/issues.yml` file on your repository and edit like below.
 
-
 ### Repository-based project
 
 ```yml
@@ -73,6 +72,10 @@ User-based project is not supported yet
 
 ## Configurations
 
+### Docker images
+
+Docker images for this action are automatically built when master branch is updated or new release tags are created in this repository with `latest` and corresponded tag names (e.g: `v.0.0.1`). Please use `uses: docker://takanabe/github-actions-automate-projects:v0.0.1` and `uses: docker://takanabe/github-actions-automate-projects:latest` syntax to use pre-build images for this GitHub Actions.
+
 ### Environment variables
 
 | Environment variable       | Value                                                                                                                                       | Description                                                                                                                                                                                                                                                                                                                                                                                                                                    |