Add GitHub Issue template

[Carles Sala]

A template should be added for the github issues, such as:

* SteganoGAN version or git commit:
* Python version (output of `pip --version`):
* Pip version (output of `pip --version`):
* Operating System:

### Description

Describe what you were trying to get done.
Tell us what happened, what went wrong, and what you expected to happen.

### What I Did

```
Paste (all) the command(s) you ran and the output.
If there was a crash, please include the traceback here.
```