feat: add readme (#7)

README.md

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+# send-google-chat-webhook
+
+This github action will enable users to send notification to google chat via github actions.
+
+## Prerequisites
+### Obtain a web hook from your google chat workspace.
+1. Go to your chat space which you want to add a webhook.
+2. At the top, click on the space title, select Apps & Integration.
+3. Click Manage webhooks
+4. Create a webhook or add another webhook if there is already one.
+5. Copy the webhook URL that you intend to use for this github action.
+
+For your own security purposes, we would suggest to store your webhook url in github secrets, and use `${{ secrets.WEBHOOK_URL}}` to get it's value.
+
+## Usage
+
+```yaml
+jobs:
+  job_id:
+    # ...
+
+    permissions:
+      contents: 'read'
+      id-token: 'write'
+
+    steps:
+    # ...
+
+    - id: 'notify google chat'
+      uses: 'google-github-actions/sent-google-chat-webhook@v0.0.1'
+      with:
+        webhook_url: '${{ secrets.WEBHOOK_URL }}'
+        mention: "<users/all>"
+```
+
+You can customize the condition for when you want this action is called..
+
+```yaml
+- id: 'notify google chat'
+  if: ${{ inputs.fail_intentionally }}
+  uses: 'google-github-actions/sent-google-chat-webhook@v0.0.1'
+  with:
+    webhook_url: '${{ secrets.WEBHOOK_URL }}'
+    mention: "<users/all>"
+```
+
+Helpful references:
+* Messages and Cards
+  * [Create, read, update, delete messages](https://developers.google.com/chat/api/guides/crudl/messages)
+  * [Send a card message](https://developers.google.com/chat/api/guides/message-formats/cards)
+  * [REST Resource: spaces.messages](https://developers.google.com/chat/api/reference/rest/v1/spaces.messages)
+  * [Method: spaces.messages.create](https://developers.google.com/chat/api/reference/rest/v1/spaces.messages/create)
+  * [Cards v2](https://developers.google.com/chat/api/reference/rest/v1/cards)
+* abcxyz
+  * [abcxyz/pkg/cli](https://pkg.go.dev/github.com/abcxyz/pkg/cli)