Create README for settings service

The just moves over the contents from  docs/services/settings/_index.md.

docs/services/settings/_index.md

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----
-title: "Settings"
-date: 2018-05-02T00:00:00+00:00
-weight: 20
-geekdocRepo: https://github.com/owncloud/ocis
-geekdocEditPath: edit/master/docs/services/settings
-geekdocFilePath: _index.md
-geekdocCollapseSection: true
----
-
-## Abstract
-
-When using oCIS, the requirement to store settings arises. This extension provides functionality
-for other extensions to register new settings within oCIS. It is responsible for storing the respective
-settings values as well.
-
-{{< mermaid class="text-center">}}
-graph TD
-    ows ---|"listSettingsBundles(),<br>saveSettingsValue(value)"| os[ocis-settings]
-    owc ---|"listSettingsValues()"| sdk[oC SDK]
-    sdk --- sdks{ocis-settings<br>available?}
-    sdks ---|"yes"| os
-    sdks ---|"no"| defaults[Use set of<br>default values]
-    oa[oCIS services<br>e.g. ocis-accounts] ---|"saveSettingsBundle(bundle)"| os
-{{< /mermaid >}}
-
-The diagram shows how the settings service integrates into oCIS:
-
-**Settings management:**
-- oCIS services can register *settings bundles* with the ocis-settings service.
-
-**Settings usage:**
-- Extensions can query ocis-settings for *settings values* of a user.
-
-## Table of Contents
-
-{{< toc-tree >}}