some ngen build instructions

NGEN_EXAMPLE.md

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+# Running Snow17 in NextGen with Sample Data
+
+See general [instructions](https://github.com/NOAA-OWP/ngen/wiki/NGen-Tutorial#running-cfe) for building models in the ngen framework. 
+
+### Build
+```
+git clone https://github.com/noaa-owp/ngen && cd ngen
+git submodule update --init --recursive
+```
+
+  - #### CFE
+    ```
+    git submodule update --remote extern/cfe/cfe 
+    cmake -B extern/cfe/cmake_build -S extern/cfe/cfe/ -DNGEN=ON
+    make -C extern/cfe/cmake_build
+    ```  
+  - #### fortran bmi
+    ```
+    cmake -B extern/iso_c_fortran_bmi/cmake_build -S extern/iso_c_fortran_bmi
+    make -C extern/iso_c_fortran_bmi/cmake_build
+    ```
+  - #### Snow17
+    ```
+    git clone https://github.com/NOAA-OWP/snow17.git ./extern
+    cmake -B extern/snow17/cmake_build -S extern/snow17
+    cmake --build extern/snow17/cmake_build --target all
+    ``` 
+  - #### PET
+    ```
+    cmake -B extern/evapotranspiration/cmake_build -S extern/evapotranspiration/evapotranspiration/
+    make -C extern/evapotranspiration/cmake_build/
+    ```
+  - #### Build [SLoTH](https://github.com/NOAA-OWP/SLoTH) using the following instructions. SLoTH bmi provides dummy values for bmi input variables that are not used in the realization.
+    ```
+    cd extern/sloth/ && git checkout latest 
+    git submodule update --init --recursive
+    cd ../..
+    cmake -B extern/sloth/cmake_build -S extern/sloth/
+    make -C extern/sloth/cmake_build
+    ```
+  - #### NGEN
+  ```
+  cmake -B cmake_build -S . -DBMI_C_LIB_ACTIVE=ON -DBMI_FORTRAN_ACTIVE=ON
+  make -j4 -C cmake_build
+  ```
+  
+ ### Run 
+Pre-process step for configuration and data pathing
+ ```
+  tar -xzvf extern/snow17/test_cases/ex1.tgz extern/snow17/test_cases/
+  cp extern/snow17/configs/example_bmi_multi_realization_config_w_snow17.json data
+  cp extern/snow17/configs/snow17-init-cat-27.namelist.input data/bmi/fortran/snow17-init-cat-27.namelist.input
+  ```
+Integrated models (SLoTH+PET+Snow17+CFE) example 
+ ```
+./cmake_build/ngen data/catchment_data.geojson cat-27 data/nexus_data.geojson nex-26 data/example_bmi_multi_realization_config_w_snow17.json
+ ```
+