update: Add the link of the guide to get additional storage for Proto…

…n Drive

Signed-off-by: qiyongzheng <[email protected]>

docs/cloud.md

@@ -27,7 +27,7 @@ Nextcloud is [still a recommended tool](document-collaboration.md#nextcloud) for
 
 ![Proton Drive logo](assets/img/cloud/protondrive.svg){ align=right }
 
-**Proton Drive** is an encrypted cloud storage provider from the popular encrypted email provider [Proton Mail](email.md#proton-mail). The initial free storage is limited to 2GB, but with the completion of certain steps, additional storage can be obtained up to 5GB.
+**Proton Drive** is an encrypted cloud storage provider from the popular encrypted email provider [Proton Mail](email.md#proton-mail). The initial free storage is limited to 2GB, but with the completion of [certain steps](https://proton.me/support/more-free-storage-existing-users), additional storage can be obtained up to 5GB.
 
 [:octicons-home-16: Homepage](https://proton.me/drive){ .md-button .md-button--primary }
 [:octicons-eye-16:](https://proton.me/drive/privacy-policy){ .card-link title="Privacy Policy" }