Merge branch 'docs-add-snippets-api-index' into 'master'

Link to Snippets API docs from API index

See merge request gitlab-org/gitlab-ce!18536

doc/api/README.md

@@ -13,6 +13,7 @@ following locations:
 - [Broadcast Messages](broadcast_messages.md)
 - [Project-level Variables](project_level_variables.md)
 - [Group-level Variables](group_level_variables.md)
+- [Code Snippets](snippets.md)
 - [Commits](commits.md)
 - [Custom Attributes](custom_attributes.md)
 - [Deployments](deployments.md)