nodefs: add README.md to document decisions and rationales.

nodefs/README.md

+
+Objective
+=========
+
+A high-performance FUSE API that minimizes pitfalls with writing
+correct filesystems.
+
+Decisions
+=========
+
+   * Nodes contain references to their children. This is useful
+     because most filesystems will need to construct tree-like structures.
+
+   * Nodes can be "persistent", meaning their lifetime is not under
+     control of the kernel. This is useful for constructing FS trees
+     in advance.
+
+   * The NodeID (used for communicating with kernel) is equal to
+     Attr.Ino (value shown in Stat and Lstat return values.) 
+
+   * No global treelock, to ensure scalability.
+
+   * Immutable characteristics of the Inode are passed on
+     creation. These are {NodeID, Mode}. Files cannot change type
+     during their lifetime. It also prevents the common error of
+     forgetting to return the filetype in Lookup/GetAttr.
+
+To decide
+=========
+
+   * Should we provide automatic fileID numbering?
+   
+   * Should OpenDir/ReadDir read the entire directory in one go?
+
+   * One giant interface with many methods, or many one-method interfaces?
+ 
+   * one SetAttr method, or many (Chown, Truncate, etc.)
+
+   * function signatures, or types? The latter is easier to remember?
+     Easier to extend?
+
+```
+    func Lookup(name string, out *EntryOut) (Node, Status) {
+    }
+
+
+    type LookupOp struct {
+      // in
+      Name string
+
+      // out
+      Child Node
+      Out *EntryOut
+    }
+    func Lookup(op LookupOp)
+```
+
+   * What to do with semi-unused fields (CreateIn.Umask, OpenIn.Mode, etc.)
+   
+   * cancellation through context.Context (standard, more GC overhead)
+     or a custom context (could reuse across requests.)?
+
+