adding documentation for link protocols

Author: jonassorgenfrei

docs/howto/customize-link-protocols.md

@@ -0,0 +1,34 @@
+# Customizing Link Protocols
+
+The Zulip app supports opening certain link protocols directly in their associated system applications. These are known as **whitelisted protocols**.
+
+## Default Whitelisted Protocols
+
+By default, the following protocols are whitelisted:
+
+```
+http https mailto tel sip
+```
+
+Links using these protocols are opened directly by the system.
+
+All other protocols are considered potentially unsafe and are therefore opened indirectly—through a local HTML file—in your default web browser.
+
+## Extending the Whitelisted Protocols
+It is possible to customize the list of whitelisted protocols by editing the `settings.json` file located at: `userdata/Zulip/config/settings.json`
+
+To add or modify the list, the `whitelistedProtocols` key can be updated. For example:
+
+```json
+{
+    ...
+    "whitelistedProtocols": [
+        "http:",
+        "https:",
+        "mailto:"
+    ]
+    ...
+}
+```
+
+Note: Each protocol should include the trailing colon (:), e.g., "mailto:" instead of "mailto".