Add documentation for the 'request access to project' feature

Signed-off-by: Rémy Coutable <remy@rymai.me>

doc/workflow/add-user/add-user.md

@@ -87,3 +87,25 @@ invitation, change their access level or even delete them.
 
 Once the user accepts the invitation, they will be prompted to create a new
 GitLab account using the same e-mail address the invitation was sent to.
+
+## Request access to a project
+
+As a user, you can request to be a member of a project. Go to the project you'd
+like to be a member of, and click the **Request Access** button on the right
+side of your screen.
+
+![Request access button](img/request_access_button.png)
+
+---
+
+Project owners & masters will be notified of your request and will be able to approve or
+decline it on the members page.
+
+![Manage access requests](img/access_requests_management.png)
+
+---
+
+If you change your mind before your request is approved, just click the
+**Withdraw Access Request** button.
+
+![Withdraw access request button](img/withdraw_access_request_button.png)