ci: disable readme_stars.schedule by default

reason: if enough people use the default cron, this results in traffic burst.

document the need to change the schedule.

.github/workflows/readme-stars.yml

@@ -1,8 +1,9 @@
 name: Update readme ⭐️ progress
 
 on:
-    schedule:
-        - cron: "51 */4 * * *" # Every 4 hours
+    # !Please set a different minute than 51 if you enable this!
+    # schedule:
+    #     - cron: "51 */6 * * *" # Every 6 hours
     workflow_dispatch:
 
 jobs:

README.md

@@ -169,6 +169,8 @@ Go to the _Secrets_ tab in your repository settings and create the following sec
 -   `AOC_YEAR`: the year you want to track. Example: `2021`
 -   `AOC_SESSION`: an active session[^2] for the advent of code website. To get this, press F12 anywhere on the Advent of Code website to open your browser developer tools. Look in your Cookies under the Application or Storage tab, and copy out the `session` cookie.
 
+✨ You can now run this action manually via the _Run workflow_ button on the workflow page. If you want the workflow to run automatically, uncomment the `schedule` section in the `readme-stars.yml` workflow file or add a `push` trigger.
+
 ### Use VS Code to debug your code
 
 1.  Install [rust-analyzer](https://marketplace.visualstudio.com/items?itemName=rust-lang.rust-analyzer) and [CodeLLDB](https://marketplace.visualstudio.com/items?itemName=vadimcn.vscode-lldb).