docs: update DEVELOPERS.md with info on conventional commits
docs: update DEVELOPERS.md with info on conventional commits

We're now using and enforcing conventional commits in the SDK repo. To inform external developers,
information about how this works is added to the DEVELOPERS.md file.

Ticket: BG-25945

BitGo/BitGoJS,BitGo/BitGoJS,BitGo/BitGoJS,BitGo/BitGoJS