Add fabricbot readme. (#14801)

Copied from https://github.com/dotnet/maui/blob/main/.github/fabricbot_readme.md.

.github/fabricbot_readme.md

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+FabricBot readme
+================
+
+FabricBot is an automated tool that responds to changes in issues, PRs, or
+runs on a schedule. It can modify issues and PRs by adding/removing labels,
+adding comments, or performing other operations.
+
+Those rules are all defined in the associated
+[fabricbot.json](/.github/fabricbot.json) file in this folder.
+
+While you can _try_ to edit this file manually, you can instead use the Fabric
+Bot editor UI to modify the file in a more reliable manner:
+
+1. Download the [fabricbot.json](/.github/fabricbot.json) file to your local disk
+2. Go to https://portal.fabricbot.ms/bot/ (don't log in!)
+3. Click **Choose File** and select the JSON file that you downloaded
+4. And press Submit to load the file and launch the FabricBot rules editor
+5. In this UI you can:
+   - View all existing rules
+   - Click **Add Task** in the upper-right corner to add a new task. This is
+     where you can see the various capabilities of FabricBot
+6. And then click around in the tool to see the various abilities and options
+
+Don't worry, you can click all you want, because that UI won't save anything
+to anywhere. You can export the rules and download the generated JSON file.
+But don't worry about accidentally changing the FabricBot rules in this repo 😊
+
+If you want to save your changes, click the **Export configuration** file, and
+upload the new file to the repo's rules at `/.github/fabricbot.json`.