Added Documentation for Zenvia integration (RocketChat#1684)

Author: richieri

administrator-guides/livechat/whatsapp-zenvia/README.md

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+# Rocket.Chat and Zenvia API integration
+
+**Added in version:**
+
+3.1.0-develop
+
+**Type of feature:**
+
+SMS Integration
+
+**Involved settings:**
+
+SMS -> Zenvia -> API Token: Authentication Token for Zenvia API
+
+SMS -> Zenvia -> Channel: Channel to be used with Zenvia API. Possible values are whatsapp, sms or facebook
+
+SMS -> Zenvia -> From: Default "From" name for API calls
+
+**Description:**
+
+This feature is almost based on Twilio's integration, making it possible to integrate Omnichannel with Whatsapp, Facebook or SMS through Zenvia.
+
+**How to use this feature:**
+
+## Step 1
+
+Ask access to the [Zenvia Sandbox](https://app.zenvia.com/home/sandbox).
+
+## Step 2
+
+Create a new Sandbox choosing **Whatsapp** channel.
+
+## Step 3
+
+Setup your whatsapp as asked to attach the sandbox.
+
+## Step 4
+
+In the final step, set **"Message Webhook URL"** to:
+
+`https://<SERVER_URL>/api/v1/livechat/sms-incoming/zenvia`
+
+## Step 5
+
+Fill the settings of Rocket.Chat at SMS administration area, under Zenvia subsection.
+
+
+That's it!