Create DESIGN.md

DESIGN.md

+# RedoxFS Design Document
+
+## Structures
+
+### Header
+The header is the entry point for the filesystem. When mounting a disk or image, it should be scanned for a block starting with the 8-byte signature, within the first megabyte:
+```rust
+"RedoxFS\0"
+```
+
+The header stores the filesystem version, disk identifier, disk size, root block pointer, and free block pointer.
+
+```rust
+#[repr(packed)]
+pub struct Header {
+    pub signature: [u8; 8],
+    pub version: u64,
+    pub uuid: [u8; 16],
+    pub size: u64,
+    pub root: u64,
+    pub free: u64,
+}
+```
+
+The root and free block pointers point to a Node that identifies
+
+### Node
+
+```
+#[repr(packed)]
+pub struct Node {
+    pub name: [u8; 256],
+    pub mode: u64,
+    pub next: u64,
+    pub extents: [Extent; 15],
+}
+```