Activity Diagram to Flow.md

[eviterin]

Context (Problem, Motivation, Solution)

IMO: Logic will only get more complicated. Better to start documenting it now, and keep it up to date.

Describe Your Changes

Added a diagram flow for New Game in flow.md.
Added a folder for assets in docs folder.
Added the UML (both .png and .uml) in the assets folder.

Checklist

• I have performed a self-review of my code
• I ran make check and fixed resulting issues
  I get warnings. Is that fine?
• I ran the relevant tests and they all pass
• I wrote tests for my new features, or added regression tests for the bug I fixed

Testing

I manually checked that flow.md was rendering the expected image.

[norswap]

Seems good! Last ask, can you avoid reformating the indentation on the markdown file?

[eviterin]

I think I fixed the indentation you were talking about.