docs(readme): add instruction

Author: exuanbo

README.md

@@ -1,19 +1,41 @@
 # Deploy to Gist
 
-[![test](https://github.com/exuanbo/actions-deploy-gist/workflows/test/badge.svg?branch=main&event=push)](https://github.com/exuanbo/actions-deploy-gist/actions?query=workflow%3Atest)
+[![GitHub Workflow Status (branch)](https://img.shields.io/github/workflow/status/exuanbo/actions-deploy-gist/test/main?event=push&label=test&logo=github&style=flat-square)](https://github.com/exuanbo/actions-deploy-gist/actions?query=workflow%3Atest)
 
-## Getting started
+## Quick start
 
 ```yml
+- uses: actions/checkout@v2
 - name: Deploy
   uses: exuanbo/actions-deploy-gist@v1
   with:
     token: ${{ secrets.TOKEN }}
-    gist_id: if_you_don't_have_then_create_one
+    gist_id: from_your_gist_url
     gist_file_name: foo.bar
     file_path: ./dist/foo.bar
 ```
 
+## Setup
+
+### Prep work
+
+1. Create a public gist if you don't have one.
+1. Generate a new [Personal access token](https://github.com/settings/tokens/). Only the `gist` scope is needed.
+
+Check [Scopes for OAuth Apps](https://docs.github.com/en/developers/apps/scopes-for-oauth-apps) for details.
+
+### Project setup
+
+1. Edit your repo `.github/workflows/foo.yml`.
+1. Go to your repo **Settings > Secrets**. Add the token generated above as `TOKEN`.
+
+### Options
+
+- `token`: `${{ secrets.TOKEN }}`
+- `gist_id`: The id portion from your gist url, e.g. `https://gist.github.com/exuanbo/`**`e885afa349a0e5d1cfb408e46d6a37bc`**.
+- `gist_file_name`: Name of the file to be added in your gist. If not provided, the original file name from `file_path` will be used.
+- `file_path`: E.g. `./dist/foo.bar`.
+
 ## License
 
 [MIT](https://github.com/exuanbo/actions-deploy-gist/blob/master/LICENSE)