Add info about MERGE command to postgresql roadmap

[kamilglod]

Roadmap URL

https://roadmap.sh/postgresql-dba

Suggestions

Here is detailed docs about MERGE feature https://www.postgresql.org/docs/current/sql-merge.html
I guess there are some articles around this topic available.
The best place is probably somewhere in "DML Queries" section

[dansholds]

Hey @kamilglod!

That sounds good to me, feel free to submit a PR adding this section to - https://github.com/kamranahmedse/developer-roadmap/blob/master/src/data/roadmaps/postgresql-dba/content/104-learn-sql-concepts/101-dml-queries/index.md

You can copy the other sections' style 🎉