Link to SAML docs from doc index

Without this link, the SAML docs are hard to discover.

doc/integration/README.md

@@ -7,6 +7,7 @@ See the documentation below for details on how to configure these services.
 - [External issue tracker](external-issue-tracker.md) Redmine, JIRA, etc.
 - [LDAP](ldap.md) Set up sign in via LDAP
 - [OmniAuth](omniauth.md) Sign in via Twitter, GitHub, GitLab, and Google via OAuth.
+- [SAML](saml.md) Configure GitLab as a SAML 2.0 Service Provider
 - [Slack](slack.md) Integrate with the Slack chat service
 - [OAuth2 provider](oauth_provider.md) OAuth2 application creation
 - [Gmail](gitlab_buttons_in_gmail.md) Adds GitLab actions to messages