Documenting "How to add a new model"

[tund]

The draft outline is as follows:

• Branching
• Add model main .py file to models package
• Create testing script (add demo data if needed) including:
• Sanity tests:
  • check gradients
  • Searching parameters
• Run pytest
• Push to remote branch
• Make pull request and ask other members to review and merge.

@mahossam : Could you please documenting by replying (commenting) in this issue, and then we can easily paste to https://github.com/tund/male/wiki/Developer-Start-Guide?

[tund]

Demonstrate by implementing Multilayer Perceptron model.

[tund]

Done. Thanks @mahossam