docs(nix/readTree): Add a very descriptive README for readTree r/280

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+readTree
+========
+
+This is a Nix program that builds up an attribute set tree for a large
+repository based on the filesystem layout.
+
+It is in fact the tool that lays out the attribute set of this repository.
+
+As an example, consider a root (`.`) of a repository and a layout such as:
+
+```
+.
+├── third_party
+│   ├── default.nix
+│   └── rustpkgs
+│       ├── aho-corasick.nix
+│       └── serde.nix
+└── tools
+    ├── cheddar
+    │   └── default.nix
+    └── roquefort.nix
+```
+
+When `readTree` is called on that tree, it will construct an attribute set with
+this shape:
+
+```nix
+{
+    tools = {
+        cheddar = ...;
+        roquefort = ...;
+    };
+
+    third_party = {
+        # the `default.nix` of this folder might have had arbitrary other
+        # attributes here, such as this:
+        favouriteColour = "orange";
+
+        rustpkgs = {
+            aho-corasick = ...;
+            serde = ...;
+        };
+    };
+}
+```
+
+Every imported Nix file that yields an attribute set will have a `__readTree =
+true;` attribute merged into it.
+
+## Traversal logic
+
+`readTree` will follow any subdirectories of a tree and import all Nix files,
+with some exceptions:
+
+* A folder can declare that its children are off-limit by containing a
+  `.skip-subtree` file. Since the content of the file is not checked, it can be
+  useful to leave a note for a human in the file.
+* If a folder contains a `default.nix` file, no *sibling* Nix files will be
+  imported - however children are traversed as normal.
+* If a folder contains a `default.nix` it is loaded and, if it evaluates to a
+  set, *merged* with the children. If it evaluates to anything else the children
+  are *not traversed*.
+
+Traversal is lazy, `readTree` will only build up the tree as requested. This
+currently has the downside that directories with no importable files end up in
+the tree as empty nodes (`{}`).
+
+## Import structure
+
+`readTree` is called with two parameters: The arguments to pass to all imports,
+and the initial path at which to start the traversal.
+
+The package headers in this repository follow the form `{ pkgs, ... }:` where
+`pkgs` is a fixed-point of the entire package tree (see the `default.nix` at the
+root of the depot).
+
+In theory `readTree` can pass arguments of different shapes, but I have found
+this to be a good solution for the most part.
+
+Note that `readTree` does not currently make functions overridable, though it is
+feasible that it could do that in the future.