add docs for existing API

doc/api/README.md

@@ -0,0 +1,29 @@
+# Gitlab API
+
+All API requests require authentication. You need to pass `private_token` parameter to authenticate.
+
+To get or reset your token visit your profile.
+
+If no or invalid `private_token` provided error message will be returned with status code 401:
+
+```json
+{
+  "message": "401 Unauthorized"
+}
+```
+
+API requests should be prefixed with `api` and the API version.
+API version is equal to Gitlab major version number and defined in `lib/api.rb`.
+
+Example of valid API request:
+
+```
+GET http://example.com/api/v2/projects?private_token=QVy1PB7sTxfy4pqfZM1U
+```
+
+The API uses JSON to serialize data. You don't need to specify `.json` at the end of API URL.
+
+## Contents
+
++ [Users](https://github.com/gitlabhq/gitlabhq/blob/master/doc/api/users.md)
++ [Projects](https://github.com/gitlabhq/gitlabhq/blob/master/doc/api/projects.md)