add adminer

part of awesome-selfhosted/awesome-selfhosted#127

README.md

@@ -507,6 +507,7 @@ Comparison of NoSQL servers: http:https://kkovacs.eu/cassandra-vs-mongodb-vs-couchdb-v
 * [Firebird](http:https://www.firebirdsql.org/) - True universal database.
 * [Galera](http:https://galeracluster.com/) - Galera Cluster for MySQL is an easy-to-use high-availability solution with high system up-time, no data loss, and scalability for future growth.
 * [MariaDB](https://mariadb.org/) - Community-developed fork of the MySQL.
+ * [adminer](http:https://www.adminer.org/) - Database management in a single PHP file ([Source code](https://github.com/vrana/adminer))
 * [Percona Server](http:https://www.percona.com/software) - Enhanced, drop-in MySQL replacement.
 * [PostgreSQL](http:https://www.postgresql.org/) - Object-relational database management system (ORDBMS).
 * [PostgreSQL-XL](http:https://www.postgres-xl.org/) - Scalable PostgreSQL-based database cluster.