Create README for settings service

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

services/settings/README.md

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+# Settings
+
+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.