Add 3rd stage docs

3-TrainMogonet/README.md

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+## Train Mogonet
+
+Having created the input matrices, we should feed the data to Mogonet architecture. Following these steps:
+
+1. Clone the repository: (preferably in Colab or a server with GPU available)
+
+```bash
+git clone https://github.com/txWang/MOGONET.git
+```
+
+2. Configure the code. To do so, copy the output from previous stage to the `MOGONET` directory and add the directory name and info to `main_biomarker.py` and `main_mogonet.py`.
+
+3. Run `main_mogonet.py` to train the network and store the model
+
+4. Run `main_biomarker.py` to evaluate the feature importances and extract biomarkers from the model.