docs(lib): init readme with folder and testing conventions

lib/README.md

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+# ClanLib
+
+This folder is supposed to contain clan specific nix functions.
+
+Such as:
+
+- build-clan function
+- select
+- build-inventory function
+- json-schema-converter
+
+## Structure
+
+Similar to `nixpkgs/lib` this produces a recursive attribute set in a fixed-point.
+Functions within lib can depend on each other to create new abstractions.
+
+### Conventions
+
+Note: This is not consistently enforced yet.
+If you start a new feature, or refactoring/touching existing ones, please help us to move towards the below illustrated.
+
+A single feature-set/module may be organized like this:
+
+```nix
+#     ↓ The final clanLib
+{lib, clanLib, ...}:
+# ↓ portion to add to clanLib
+{
+    inventory.resolveTags = tags: inventory.machines; # implementation
+    inventory.buildMachines = x: clanLib.inventory.resolveTags x; # implementation
+}
+```
+
+Every bigger feature should live in a subfolder with the feature name.
+It should contain two files:
+
+- `impl.nix`
+- `test.nix`
+- Everything else may be adopted as needed.
+
+```
+Example filetree
+```
+```sh
+.
+├── default.nix
+├── feature_foo
+│   ├── impl.nix
+│   └── test.nix
+└── feature_bar
+    ├── impl.nix
+    ├── complex-subfeature
+    │   ├── impl.nix
+    │   └── test.nix
+    ├── testless-subfeature # <- We immediately see that this feature is not tested on itself.
+    │   └── impl.nix
+    └── test.nix
+```
+
+```nix
+# default.nix
+{lib, clanLib, ...}:
+{
+    inventory.resolveTags = import ./resolveTags { inherit lib clanLib; };
+}
+```
+
+## Testing
+
+For testing we use [nix-unit](https://github.com/nix-community/nix-unit)
+
+TODO: define a helper that automatically hooks up `tests` in `flake.legacyPackages` and a corresponding buildable `checks` attribute