Adding Mecena to the adopters (#2580)

ADOPTERS.md

@@ -13,3 +13,4 @@ Thank you for trusting the `go-feature-flag` and using it in your organization.
 | Stacklok             | https://github.com/stacklok/                          | Feature flags (looking at using relay)         |
 | HelloWorldCS         | https://helloworldcs.org                              | Feature flags for internal tools + app         |
 | Alternative Payments | https://www.alternativepayments.io                    | Feature Flagging for frontend and backend apps |
+| Mecena               | https://mecena.co                                     | FF for backend microservices                   |