Added docs for group_share_group resource

Author: btyy77c

docs/resources/group_share_group.md

@@ -0,0 +1,34 @@
+# gitlab\_group\_share\_group
+
+This resource allows you to share a group with another group
+
+## Example Usage
+
+```hcl
+resource "gitlab_group_share_group" "test" {
+  group_id       = gitlab_group.foo.id
+  share_group_id = gitlab_group.bar.id
+  group_access 	 = "guest"
+  expires_at     = "2099-01-01"
+}
+```
+
+## Argument Reference
+
+The following arguments are supported:
+
+* `group_id` - (Required) The id of the main group.
+
+* `share_group_id` - (Required) The id of an additional group which will be shared with the main group.
+
+* `group_access` - (Required) One of five levels of access to the group.
+
+* `expires_at` - (Optional) Share expiration date. Format: `YYYY-MM-DD`
+
+## Import
+
+GitLab group shares can be imported using an id made up of `mainGroupId:shareGroupId`, e.g.
+
+```
+$ terraform import gitlab_group_share_group.test 12345:1337
+```