feat(kb/slack): start slack's article

knowledge base/slack.md

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+# Slack
+
+1. [TL;DR](#tldr)
+1. [Add custom emoji](#add-custom-emoji)
+1. [Give aliases to existing emojis](#give-aliases-to-existing-emojis)
+1. [Further readings](#further-readings)
+   1. [Sources](#sources)
+
+## TL;DR
+
+<details>
+  <summary>Setup</summary>
+
+```sh
+brew install --cask 'slack'
+mas install '803453959'
+```
+
+</details>
+
+<details>
+  <summary>Usage</summary>
+
+```sh
+# Send notifications to channels
+curl -X POST -H 'Content-type: application/json' \
+  --data '{"text": "Staging DB restore completed successfully"}' \
+  'https://hooks.slack.com/services/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'
+```
+
+</details>
+
+<!-- Uncomment if used
+<details>
+  <summary>Real world use cases</summary>
+
+```sh
+```
+
+</details>
+-->
+
+## Add custom emoji
+
+Check out [slackmojis] for some common reactions.
+
+1. Go to <https://{{org-name}}.slack.com/customize/emoji>.
+1. Select _Add Custom Emoji_.
+1. Upload the image.
+   Supported formats: GIF, JPG, PNG.
+1. Give it a name.
+
+## Give aliases to existing emojis
+
+1. Go to <https://{{org-name}}.slack.com/customize/emoji>.
+1. Select _Add Alias_.
+1. Choose the image.
+1. Give it an alias.
+
+## Further readings
+
+- [Website]
+
+### Sources
+
+- [Slackmojis]
+
+<!--
+  Reference
+  ═╬═Time══
+  -->
+
+<!-- In-article sections -->
+<!-- Knowledge base -->
+<!-- Files -->
+<!-- Upstream -->
+[website]: https://slack.com/
+
+<!-- Others -->
+[slackmojis]: https://slackmojis.com/