Extend documentation on organization and how to run (#42)

Co-authored-by: Florine de Geus <[email protected]>

Author: hahnjo

README.md

@@ -11,6 +11,14 @@ Eventually, we will also provide a set of reference `.root` and `.json` files (s
 > Its structure and implementation may change during development.
 > We welcome early feedback on the current state from any interested party.
 
+## Test Categories
+
+Tests in the RNTuple Validation Suite are organized into (nested) *categories*.
+This is mirrored by the hierarchical directory layout in the repository.
+For example, the [`types`](types) directory contains tests related to type support in the RNTuple specification.
+It has subdirectories for tests concerning [fundamental types](types/fundamental) and C++ types, for example [`std::vector`](types/vector).
+More tests are planned in the future, please [consult the list of issues](https://github.com/root-project/rntuple-validation/issues) in the GitHub repository.
+
 ## Reference Implementation
 
 This repository also contains a reference implementation with ROOT macros.
@@ -19,3 +27,13 @@ In this release, the API is not yet finalized and all classes are in the `ROOT::
 We plan to rely on this version until a first version of the RNTuple Validation Suite is done.
 Then we will produce a first set of reference files that can be used to test backwards compatibility.
 Afterwards, the implementation will be updated for the stable API (expected in ROOT v6.36).
+
+### How to Run
+
+For each test, we implement a `write.C` and `read.C` macro in the corresponding subdirectory.
+The `write.C` macro produces a `.root` file with the contents as described in the `README` of each test subdirectory.
+The `read.C` macro produces a `.json` file with a human-readable representation of the data in the `.root` file.
+They can be run individually or all at once with `make` using the top-level [`Makefile`](Makefile).
+The latter is also exercised by a GitHub Actions Workflow:
+![Execute ROOT macros](https://github.com/root-project/rntuple-validation/actions/workflows/root.yml/badge.svg)
+The job also uploads the produced set of `.root` and `.json` files, which can be downloaded from the Summary page.