Add Element to Slack alternatives

replace/slack.md

+---
+product: Slack
+type: Communication platforms
+layout: replace
+---
+
+# Slack
+
+## Element
+
+All-in-one secure chat app for teams, friends and organisations. Keeps conversations in your control, [safe from data-mining and ads.](https://sifted.eu/articles/element-germany-deal/) Talk to everyone through the open global Matrix network, protected by proper end-to-end encryption.
+
+{% infobox %}
+  - **Website**: [element.io](https://element.io)
+  - **Source code**: [GitHub](github.com/vector-im/riot-web)
+  - **License**: [Apache License 2.0](https://github.com/vector-im/riot-web/blob/develop/LICENSE)
+{% endinfobox %}
+