feat: add linkedin extension (#144)

Co-authored-by: Lukasz Gornicki <[email protected]>

extensions/linkedin.md

@@ -0,0 +1,29 @@
+# Linkedin Extension
+This document defines how to use `linkedin` extension in AsyncAPI documents.
+
+## Overview 
+This extension allows you to provide the Linkedin username of the account representing the team/company of the API.
+
+## Version
+Current version is `0.1.0`.
+
+## Extension Definition
+
+### Type: String
+
+URL of the Linkedin Profile of the user or company.
+
+## Extension Location 
+
+This extension can be used in the following locations:
+- [Info Object](https://www.asyncapi.com/docs/reference/specification/v3.0.0#infoObject)
+
+## Example
+
+```yaml
+asyncapi: '3.0.0'
+info
+  title: AsyncAPI Initiative Example
+  version: '1.0.0'
+  x-linkedin: https://www.linkedin.com/company/asyncapi
+```