Add Adminer to Database Management (#3964)

- fixes https://github.com/awesome-foss/awesome-sysadmin/issues/444

README.md

@@ -729,6 +729,7 @@ _Related: [Analytics](#analytics), [Automation](#automation)_
 
 _See also: [dbdb.io - Database of Databases](https://dbdb.io/)_
 
+- [Adminer](https://www.adminer.org/) - Database management in a single PHP file. Available for MySQL, MariaDB, PostgreSQL, SQLite, MS SQL, Oracle, Elasticsearch, MongoDB and others. ([Source Code](https://github.com/vrana/adminer/)) `Apache-2.0/GPL-2.0` `PHP`
 - [Baserow](https://baserow.io/) - Open source online database tool and Airtable alternative. Create your own database without technical experience. ([Source Code](https://gitlab.com/bramw/baserow)) `MIT` `Python/Nodejs`
 - [Bytebase](https://www.bytebase.com/) - Safe database schema change and version control for DevOps teams, supports MySQL, PostgreSQL, TiDB, ClickHouse, and Snowflake. ([Demo](https://demo.bytebase.com), [Source Code](https://github.com/bytebase/bytebase)) `MIT` `Docker/Go`
 - [Chartbrew](https://chartbrew.com) - Web application that can connect directly to databases and APIs and use the data to create beautiful charts. ([Demo](https://app.chartbrew.com/live-demo), [Source Code](https://github.com/chartbrew/chartbrew)) `MIT` `Nodejs`