Write tutorial

[dpo]

No description provided.

[paraynaud]

A first version is out in docs/src/tutorial.md

[dpo]

Please format the code in the tutorial like the rest of the source code:

struct Conv; w; b; f; end

should be

struct Conv
  w  # document what this field is
  b  # document what this field is
  f  # document what this field is
end

It's a tutorial. It should be easy to read and understand.

Conv(w1,w2,cx,cy,f=relu) = Conv(param(w1,w2,cx,cy), param0(1,1,cy,1), f)

should have spaces between arguments. Etc, etc.

[dpo]

@paraynaud