add MariaDB to supported databases in README.md

[robertsilen]

This PR suggests to mention MariaDB as a supported database in the README, as MariaDB is de facto supported as per discussion #7346

[sailsbot]

Thanks for submitting this pull request, @robertsilen! We'll look at it ASAP.

In the mean time, here are some ways you can help speed things along:

• discuss this pull request with other contributors and get their feedback. (Reactions and comments can help us make better decisions, anticipate compatibility problems, and prevent bugs.)
• ask another JavaScript developer to review the files changed in this pull request. (Peer reviews definitely don't guarantee perfection, but they help catch mistakes and enourage collaborative thinking. Code reviews are so useful that some open source projects require a minimum number of reviews before even considering a merge!)
• if appropriate, ask your business to sponsor your pull request. (Open source is our passion, and our core maintainers volunteer many of their nights and weekends working on Sails. But you only get so many nights and weekends in life, and stuff gets done a lot faster when you can work on it during normal daylight hours.)
• make sure you've answered the "why?" (Before we can review and merge a pull request, we feel it is important to fully understand the use case: the human reason these changes are important for you, your team, or your organization.)

---

Please remember: never post in a public forum if you believe you've found a genuine security vulnerability. Instead, disclose it responsibly.

For help with questions about Sails, click here.