All the info to return codes from the API functions are available in the `README.md` file as suggested.

Allow connection to Redis via unix socket, using
unix:/var/run/redis/redis.sock for example.

Default behaviour does not change, except that the full Redis URL must
be configured, with redis:// for tcp or unix: for unix socket.

Remove prompt $ from code line for uniformity

Added a helper method to check if required parameters are given in an API call. Can be used
to return a `400 Bad Request` return code if a required attribute is missing.
Code clean up and fixed tests.

The API documentation of merge requests contains info to status codes for all functions.
Required arguments are now checked in the merge requests API functions and a `400 Bad Request` error is
returned if they are not given.

The issues documentation is updated with infos to status codes and the deprecated `DELETE` function and
how to close an issue. A few more tests added to check status codes of API functions.

Updates the API documentation of groups with infos to return codes. The function calls
in the groups API have updated documentation and return `400 Bad Request` status code
if a required attribute is missing.

The API documentation of repository is updated and now contains infos to status codes.
Code documentation is also adjusted for `GET /projects/:id/repository/commits` and includes infos to
pagination attributes. Tests are updated.

The API Documentation for project snippets got infos to return codes. Tests are added
to check status codes when handling project snippets.

Updated the milestones API documentation and added return codes descriptions.

The notes API documentation updated with return codes. API now returns `400 Bad Request` if
required attributes are not present. Return codes are documented now, also tested in added tests.
The documentation now reflects the current state of the API.

The users API updated with return codes, e.g. if required parameters are missing
a `400 Bad Request` error is returned instead of `404`. Fixes return codes of functions,
e.g. deletion of a ssh key is an idempotent function now.
The API documentation is updated to reflect the current status of the API. Descriptions
are more detailed and complete, infos to return values are added to all functions.

changed installation overview list step #4
Gitolite -> GitLab shell 

Information to return codes added to projects and users documentation.

Added creation of pids directory to fix #2995, replaced deprecated nginx init.d command with service one, some more clean-up

Documentation is updated with information how to handle snippets or how to access tags
and commits. Nearly all project specific functions are now described in the documentation.
A few previous entries have been updated with status codes, e.g. `401 Unauthorized`.

The API documentation for projects now is structured into major sections that describe
the different aspects when dealing with projects, e.g. hooks, branches, team members etc.
All described methods now contain a list of possible status codes the method can return. A few
methods have extra sample JSON responses and a description if a method is idempotent.
Idempotent methods can be called multiple times while returning the same status code.

Instead of using funtion `error!` the function `not_found!` is used to return 404 error. Adjusted documentation accordingly.

A few fixes in the notes URI pattern (`notes` instead of `:notes`), also updated
the information to status codes. If `body` attribute is missing from a POST request
a status code 400 (Bad request) is returned. This reflects the code changes from
the previous commit.