docs: note MariaDB support in README

[robertsilen]

Summary

Documents that CloudStack uses a MySQL-compatible database. The Docker development setup in tools/docker/docker-compose.yml uses MariaDB (mariadb:10.0); the README only mentioned MySQL in the cryptographic software notice.

Documentation only; no code or deployment changes.

Test plan

• README only

[boring-cyborg]

Congratulations on your first Pull Request and welcome to the Apache CloudStack community! If you have any issues or are unsure about any anything please check our Contribution Guide (https://github.com/apache/cloudstack/blob/main/CONTRIBUTING.md)
Here are some useful points:

• In case of a new feature add useful documentation (raise doc PR at https://github.com/apache/cloudstack-documentation)
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