Issue #8276 "README Markdown does not support internal links" was marked as resolved. However, the resolution shown is not a resolution, as it is counter to common industry practice, rendering Markdown files for Bitbucket incompatible with any other tool on the market.
The comments on the original issue elaborate on the necessity for cross-compatibility and offer an improvement on the bitbucket Markdown parser that would render it compatible with other common tools.
As for me, it necessitates maintaining two copies of documentation. It's easy to automate, but it would make sense to lift this burden from your customers, given that the fix should not be overly complex.