Add LinkedIn (#22)

* Add LinkedIn

* Change categories element from array to string

Author: Carlgo11

entries/l/linkedin.com.json

@@ -0,0 +1,7 @@
+{
+  "LinkedIn": {
+    "passwordless": "allowed",
+    "documentation": "https://www.linkedin.com/help/linkedin/answer/a1621596",
+    "categories": "social"
+  }
+}