docs: add guide in README.md

Signed-off-by: seolmin <seolmin@megazone.com>

Author: seolmin

README.md

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+## plugin-google-cloud-log-mon-datasource
+
+![Google Cloud Services](https://spaceone-custom-assets.s3.ap-northeast-2.amazonaws.com/console-assets/icons/cloud-services/google_cloud/cloud_logging.png)
+
+**Plugin to collect log data of Google Cloud logging**
+
+>Cloudforet's [plugin-google-cloud-log-mon-datasource](https://github.com/cloudforet-io/plugin-google-cloud-log-mon-datasource)
+is a convenient tool
+> to get Cloud Logging log data from Google.
+
+
+Find us also at [Dockerhub](https://hub.docker.com/repository/docker/spaceone/plugin-google-cloud-log-mon-datasource)
+> Latest stable version : 1.0.1
+
+Please contact us if you need any further information. (<support@spaceone.dev>)
+
+---
+
+<br>
+<br>
+
+## Google Service Endpoint (in use)
+
+There is an endpoints used to collect resources information of GCP.  
+Endpoint of served GCP is a URL consisting of a service code.
+
+```text
+https://[service-code].googleapis.com
+```
+
+<br>
+<br>
+
+## Authorization Scopes
+
+Requires one of the following OAuth scopes:
+
+```text
+https://www.googleapis.com/auth/logging.read
+https://www.googleapis.com/auth/logging.admin
+https://www.googleapis.com/auth/cloud-platform.read-only
+https://www.googleapis.com/auth/cloud-platform
+```
+
+<br>
+<br>
+
+## Authentication Overview
+
+Registered service account on SpaceONE must have certain permissions to collect cloud service data  
+Please, set authentication privilege for followings:
+
+```text
+logging.logEntries.list
+logging.privateLogEntries.list
+logging.views.access
+```