Add docs for create branch via api

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

doc/api/branches.md

@@ -165,3 +165,34 @@ Parameters:
   "protected": false
 }
 ```
+
+## Create repository branch
+
+
+```
+POST /projects/:id/repository/branches
+```
+
+Parameters:
+
++ `id` (required) - The ID of a project
++  `branch_name` (required) - The name of the branch
++  `ref` (required) - Create branch from commit sha or existing branch
+
+```json
+{
+    "name": "my-new-branch",
+    "commit": {
+        "id": "8848c0e90327a0b70f1865b843fb2fbfb9345e57",
+        "message": "Merge pull request #54 from brightbox/use_fog_brightbox_module\n\nUpdate to use fog-brightbox module",
+        "parent_ids": ["fff449e0bf453576f16c91d6544f00a2664009d8", "f93a93626fec20fd659f4ed3ab2e64019b6169ae"],
+        "authored_date": "2014-02-20T19:54:55+02:00",
+        "author_name": "john smith",
+        "author_email": "john@example.com",
+        "committed_date": "2014-02-20T19:54:55+02:00",
+        "committer_name": "john smith",
+        "committer_email": "john@example.com"
+    },
+    "protected": false
+}
+```