How to add a new user or item to the model ?

Hi @maciejkula.
First: **This library is amazing**
Second: I would like to know how to add a user to the model. It's easy to add the user to the interaction matrix:
```python
interactions.loc[user_id] = 0
```
But do I need to fit the model with all the interactions or only the new one:
```python
# this
x = sparse.csr_matrix(interactions.loc[user].values)
# or this
x = sparse.csr_matrix(interactions)
model.fit(interactions=x, epochs=epoch, num_threads=n_jobs)
```

Start by reviewing the model's fit API and any existing usage documentation or examples for adding a new user or item. Clarify whether fitting accepts only the new interaction row or requires the complete interaction matrix, and document the supported workflow and its expected input.

Written by the indexing model from the issue text.