Add backlink to Flagsmith in Public APIs repository: https://github.com/public-apis/public-apis

[malthauser]

Here is the task:

Review the Development Links (https://github.com/public-apis/public-apis/tree/master#development) and understand the formatting.

Review the Contributing Guidelines (https://github.com/public-apis/public-apis/blob/master/CONTRIBUTING.md) and use their format.

Submit a Pull Request to have Flagsmith added (https://github.com/public-apis/public-apis/pulls)