bots/giphy: Document the bot.

zulip_bots/zulip_bots/bots/giphy/doc.md

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+# GIPHY bot
+
+The GIPHY bot is a Zulip bot that can fetch a GIF associated with
+a given keyword from [GIPHY](https://giphy.com/).
+
+To use the GIPHY bot, you can simply call it with `@Giphy` followed
+by a keyword, like so:
+
+```
+@Giphy hello
+```
+
+## Setup
+
+Before you can proceed further, you'll need to go to the
+[GIPHY Developers](https://developers.giphy.com/), and get a
+GIPHY API key.
+
+1. Click on the **Create an App** button on the top right corner.
+2. Click on **Create an App** under the **Your Apps** section.
+3. Enter a name and a description for your app and click on
+   **Create New App**.
+4. And you're done! You should now have a GIPHY API key.
+5. Open up `zulip_bots/bots/giphy/giphy.conf` in an editor and
+   and change the value of the `key` attribute to the API key
+   you generated above.
+
+{!running-a-bot.md!}
+
+## Usage
+
+1. `@Giphy <keyword` - This command will fetch a GIF associated
+   with the given keyword. Example usage: `@Giphy hello`:
+   ![](/static/generated/bots/giphy/assets/giphy-gif-found.png)
+
+2. If a GIF can't be found for a given keyword, the bot will
+   respond with an error message:
+   ![](/static/generated/bots/giphy/assets/giphy-gif-not-found.png)
+
+3. If there's a connection error, the bot will respond with an
+   error message:
+   ![](/static/generated/bots/giphy/assets/giphy-connection-error.png)