Add start on BMI documentation

docs/bmi.md

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+# Basic Model Interface
+The [Basic Model Interface](https://csdms.colorado.edu/wiki/BMI) is a standard way of communicating with models.
+PyStemmusScope implements the Basic Model Interface for STEMMUS_SCOPE.
+
+There are multiple ways to run the STEMMUS_SCOPE Basic Model Interface.
+The first is using a local Matlab Compiler Runtime executable file (only available for x86 Linux).
+This requires installation of MCR.
+The other option is to use the Dockerized version of the executable, available on ghcr.io/ecoextreml/stemmus_scope.
+
+For more information on each method, see the sections below.
+
+## Local executable file
+The executable file can be downloaded from the STEMMUS_SCOPE repository. More specifically [here](https://github.com/EcoExtreML/STEMMUS_SCOPE/tree/main/run_model_on_snellius/exe).
+
+To be able to run this executable, you need a Linux x86 system, along with Matlab Compiler Runtime R2023a. MCR is available [here](https://nl.mathworks.com/products/compiler/matlab-runtime.html).
+
+To use the local executable file, add the path to the executable file to the config file. E.g.:
+```
+WorkDir=/home/username/tmp/stemmus_scope
+...
+ExeFilePath=/path/to/executable/STEMMUS_SCOPE
+```
+
+Alternatively, if the environmental variable `STEMMUS_SCOPE` is configured, the BMI will use this if the ExeFilePath or DockerImage are not set in the configuration file.
+
+## Dockerized executable
+STEMMUS_SCOPE also has a Docker image available. This allows you to run the executable file without having to install MCR.
+The Docker image is available at https://ghcr.io/ecoextreml/stemmus_scope
+
+To use the Docker image, use the `DockerImage` setting in the configuration file:
+```sh
+WorkDir=/home/username/tmp/stemmus_scope
+...
+DockerImage=ghcr.io/ecoextreml/stemmus_scope:1.5.0
+```
+
+It is best to add the version tag here too (`:1.5.0`), this way the BMI will warn you if the version might be incompatible.
+
+Note that the `docker` package for python is required here. Install this with `pip install PyStemmusScope[docker]`.
+Additionally, [Docker](https://docs.docker.com/get-docker/) itself has to be installed.