Add docs

doc/api/issue_links.md

+# Issue links API
+
+## List issue relations
+
+Get a list of related issues of a given issue, sorted by the relationship creation datetime (ascending).
+Issues will be filtered according users authorization.
+
+```
+GET /projects/:id/issues/:issue_iid/links
+```
+
+Parameters:
+
+| Attribute   | Type    | Required | Description                          |
+|-------------|---------|----------|--------------------------------------|
+| `id`        | integer/string | yes      | The ID or [URL-encoded path of the project](README.md#namespaced-path-encoding) owned by the authenticated user  |
+| `issue_iid` | integer | yes      | The internal ID of a project's issue |
+
+```json
+[
+  {
+    "id" : 84,
+    "iid" : 14,
+    "issue_link_id": 1
+    "project_id" : 4,
+    "created_at" : "2016-01-07T12:44:33.959Z",
+    "title" : "Issues with auth",
+    "state" : "opened",
+    "assignees" : [],
+    "assignee" : null,
+    "labels" : [
+      "bug"
+    ],
+    "author" : {
+      "name" : "Alexandra Bashirian",
+      "avatar_url" : null,
+      "state" : "active",
+      "web_url" : "https://gitlab.example.com/eileen.lowe",
+      "id" : 18,
+      "username" : "eileen.lowe"
+    },
+    "description" : null,
+    "updated_at" : "2016-01-07T12:44:33.959Z",
+    "milestone" : null,
+    "subscribed" : true,
+    "user_notes_count": 0,
+    "due_date": null,
+    "web_url": "http://example.com/example/example/issues/14",
+    "confidential": false,
+    "weight": null,
+  }
+]
+```
+
+## Create an issue link
+
+Creates a two-way relation between an issue to another. Users must have write access on both issues
+in order to succeed.
+
+```
+POST /projects/:id/issues/:issue_iid/links
+```
+
+| Attribute   | Type    | Required | Description                          |
+|-------------|---------|----------|--------------------------------------|
+| `id`        | integer/string | yes      | The ID or [URL-encoded path of the project](README.md#namespaced-path-encoding) owned by the authenticated user  |
+| `issue_iid` | integer | yes      | The internal ID of a project's issue |
+| `target_project_id` | integer/string | yes      | The ID or [URL-encoded path of the project](README.md#namespaced-path-encoding) of a target project  |
+| `target_issue_iid` | integer/string | yes      | The internal ID of a target project's issue |
+
+
+```json
+{
+  "id": 1,
+  "source_issue_id": 2,
+  "target_issue_id": 3
+}
+```
+
+## Delete an issue link
+
+Deletes an issue link, thus removes the two-way relationship.
+
+```
+DELETE /projects/:id/issues/:issue_iid/links/:issue_link_id
+```
+
+
+| Attribute   | Type    | Required | Description                          |
+|-------------|---------|----------|--------------------------------------|
+| `id`        | integer/string | yes      | The ID or [URL-encoded path of the project](README.md#namespaced-path-encoding) owned by the authenticated user  |
+| `issue_iid` | integer | yes      | The internal ID of a project's issue |
+| `issue_link_id` | integer/string | yes      | The ID of an issue relationship |
+
+
+```json
+{
+  "id": 1,
+  "source_issue_id": 2,
+  "target_issue_id": 3
+}
+```
+