Maintain a user reference for Embulk users

[dmikurube]

Embulk has had very few documents. There are some documents at https://www.embulk.org/docs/, but they have not been maintained for a long time. We have had no bandwidths for maintain the documents.

We wanted to have someone who is/are willing to contribute in maintaining the documents, including a total renovate if wanted.

If you're interested in it, please let us know in this Discussion at first.