[Documentation] Add information about the possibility to import metadata in dataset

:warning: Disclaimer : newbie on this project as a contributor (however I'm a 2-years users of Doccano, you're product is 🔥). This issue is about **documentation** and I know you recommend making a PR first, but since my time is a bit constrained I first would like to have insights about the need of this :warning:

## Context
As an a analyst, I work a lot on text classifications that are extracted from an external platform. These text chunks have each an ID that I must have after annotation, which I could handle thanks to the metadata field when I import data.

However, as I dive through the documentation (https://doccano.github.io/doccano/tutorial/) on the section of importing dataset, I find not trace of such possibility, which can be quite useful.

I would like to add a section in the *import a Dataset* section that gives insight about how the file can handle metadata, and more widely informations that are shown nearby annotations. Am I suggesting something worth ?

### System information
- **OS Platform and Distribution (e.g., Linux Ubuntu 16.04)**: One-click heroku deployment
- **Python version**: 3.8
Improvement would be funded by Open Source Politics