Add getting started doc

Author: peter-evans

assets/example-command.png

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+# Getting started
+
+Follow this guide to get started with a working `/example` command.
+
+## Command processing setup
+
+1. Create a new repository called, for example, `slash-command-processor`.
+   This will be the repository that commands are dispatched to for processing.
+
+2. In your new repository, create the following workflow at `.github/workflows/example-command.yml`.
+
+    ```yml
+    name: example-command
+    on:
+    repository_dispatch:
+      types: [example-command]
+    jobs:
+      example:
+        runs-on: ubuntu-latest
+        steps:
+          - name: Add reaction
+            uses: peter-evans/create-or-update-comment@v1
+            with:
+              token: ${{ secrets.REPO_ACCESS_TOKEN }}
+              repository: ${{ github.event.client_payload.github.payload.repository.full_name }}
+              comment-id: ${{ github.event.client_payload.github.payload.comment.id }}
+              reaction-type: hooray
+    ```
+
+3. Create a `repo` scoped Personal Access Token (PAT) by following [this guide](https://help.github.com/en/github/authenticating-to-github/creating-a-personal-access-token-for-the-command-line).
+
+4. Go to your repository `Settings` -> `Secrets` and `Add a new secret`.
+
+   **Name**: `REPO_ACCESS_TOKEN`
+
+   **Value**: (The PAT created in step 3)
+
+Command processing setup is complete! Now we need to setup command dispatch for our `/example` command.
+
+## Command dispatch setup
+
+1. Choose a repository or create a new repository to dispatch commands from.
+   This will be the repository where issue and pull request comments will be monitored for slash commands.
+
+   In the repository, create the following workflow at `.github/workflows/slash-command-dispatch.yml`.
+
+   **Note**: Change `your-github-username/slash-command-processor` to reference your command process repository created in the [previous section](#command-processing-setup).
+
+    ```yml
+    name: Slash Command Dispatch
+    on:
+    issue_comment:
+      types: [created]
+    jobs:
+      slashCommandDispatch:
+        runs-on: ubuntu-latest
+        steps:
+          - name: Slash Command Dispatch
+            uses: peter-evans/slash-command-dispatch@v1
+            with:
+              token: ${{ secrets.REPO_ACCESS_TOKEN }}
+              commands: example
+              repository: your-github-username/slash-command-processor
+    ```
+
+2. Create a new `repo` scoped [PAT](https://help.github.com/en/github/authenticating-to-github/creating-a-personal-access-token-for-the-command-line), OR, use the one created at step 3 of the [previous section](#command-processing-setup).
+
+3. Go to your repository `Settings` -> `Secrets` and `Add a new secret`.
+
+   **Name**: `REPO_ACCESS_TOKEN`
+
+   **Value**: (The PAT created in step 2)
+
+Command dispatch setup is complete! Now let's test our `/example` command.
+
+## Testing the command
+
+1. Create a new GitHub Issue in the repository you chose to dispatch commands from.
+
+2. Add a new comment with the text `/example`.
+
+Once the command completes you should see all three reactions on your comment.
+
+![Example Command](assets/example-command.png)
+
+Now you can start to tweak the command and make it do something useful!