You can pull in other files from CLAUDE.md using the @path/to/file import syntax. Write @README or list @docs/workflow.md as a bullet, and Claude reads those files as part of your instructions.
Relative paths resolve from the file containing the import, with a maximum depth of hops. A one-time approval dialog appears on first use so you know what's being loaded.
The .claude/rules/ directory is a second way to split your instructions. I prefer it for team repos: every .md file there loads automatically, at the same priority as .claude/CLAUDE.md.
You can scope rules to paths with YAML frontmatter by adding a paths field with glob patterns. Without that field, the rules apply to every request.