add .dirmd and other metadata

README.md

@@ -138,6 +138,26 @@ Example output with `dirmd -o docs.md ~/repos/myproject --vertical-slices`:
     ├── cmd.md           ← slice: tree + contents of cmd/
     └── internal.md      ← slice: tree + contents of internal/
 
+## Instructions File (`.dirmd`)
+
+If a file named `.dirmd` exists in the root of the input directory, you can include its contents as metadata in the generated output by passing the `--instructions` flag.
+
+This is useful for injecting specific context, constraints, or instructions for LLMs processing the snapshot.
+
+- **File Name**: `.dirmd`
+- **Location**: Root of the input directory.
+- **Format**: Plain text or Markdown.
+- **Output**: Rendered as a quoted block (`> Instructions: ...`) immediately after the timestamp and commit info.
+- **Opt-in**: The file is **ignored** unless `--instructions` is explicitly passed.
+
+Example `.dirmd`:
+
+```text
+Focus on the `internal/` package for business logic.
+Ignore `cmd/` unless explicitly asked.
+Prioritize Go files over Python.
+```
+
 ## LLM-Optimized Output
 
 Output is designed for RAG ingestion and LLM context windows, not human presentation: