How do I annotate and maintain that information

How would one go about annotating the project with information which can be mined
I understand that project descriptions have limitations interms of number of characters.
README.md files are suitable but they cannot be mined as global search doesn’t seem to work.

The goal would be to annotate projects which could later be mined. Any ideas ?

That really depends on what you mean by “mined”? You could use YAML frontmatter to provide metadata (e.g. http://assemble.io/docs/YAML-front-matter.html) which will be shown in the interface for .md files.