updated documentation

README_config.md

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+# Configurations options
+
+## About MPI parallelism
+
+When talking about **MPI** with OpenMC + Python, it helps to separate two different layers:
+
+### 1. MPI at the Python level (`mpi4py`)
+
+This is the **outer** parallelism:
+
+* You run your script with:
+
+  ```bash
+  mpiexec -n 4 python depletion_mpi.py
+  ```
+* `mpi4py` handles communication between ranks (e.g. splitting burnup steps, distributing materials, etc.).
+* Each rank calls `openmc.deplete` or `openmc.run()` from Python.
+* For this you only need:
+
+  ```bash
+  python -m pip install mpi4py
+  ```
+
+  in the same virtual environment where `openmc` is installed.
+
+This works **even if OpenMC itself was built *without* MPI** — each rank just runs its own independent OpenMC process.
+
+### 2. MPI inside OpenMC (transport parallelism)
+
+This is the **inner** parallelism, in the C++ code:
+
+* Enabled at build time with:
+
+  ```bash
+  cmake .. -DOPENMC_USE_MPI=on ...
+  ```
+* Then each OpenMC run can use multiple MPI ranks for particle transport.
+* Typically you’d start OpenMC with `mpiexec` (directly or via the Python API / executor).
+
+This is useful if you want a **single calculation** to run faster using multiple ranks.
+
+### Do you need OpenMC compiled with MPI for depletion-MPI?
+
+* For a typical **depletion-MPI pattern using only `mpi4py`** (one Python rank ↔ one OpenMC process, no inner MPI):
+
+  * ❌ You do **not strictly need** OpenMC compiled with MPI.
+  * `mpiexec -n N python depletion_mpi.py` with a **serial** OpenMC build is perfectly fine.
+
+* If you want **both**:
+
+  * MPI between depletion tasks (`mpi4py`), **and**
+  * MPI *inside* each OpenMC transport solve,
+
+  then ✅ you **do** need `OPENMC_USE_MPI=on` in your OpenMC build and a careful layout of ranks/cores.
+
+### What this means for you
+
+You already have an OpenMC build with:
+
+* `MPI enabled: yes`
+
+So you’re fully covered:
+
+* To use depletion-MPI, just install `mpi4py` in your venv:
+
+  ```bash
+  python -m pip install mpi4py
+  ```
+* Run your script with:
+
+  ```bash
+  mpiexec -n 4 python your_depletion_script.py
+  ```
+
+You can choose to:
+
+* Use **only outer MPI** (`mpi4py`, each rank runs serial OpenMC), or
+* Combine outer MPI + inner MPI if you really need both levels of parallelism.
+