If you have an idea about how the project should evolve, create an issue with the tag enhancement or idea.
This issue will act like a sounding board for design. Link to pull requests in the issue to reference any design documents (as .md files), prototype code, or an implementation. Once the issue has sprouted, and has a life of its own in an implementation PR, transcribe the summary or conclusions of the design discussion into the design/ directory, and include those notes in the implementation PR.
Use Github's Linked Issues feature to automatically close the issue when the implementation PR is merged.
Markdown files are automatically rendered as a github page here.