contribution guidance

[ccoVeille]

Hi,

I opened #15 and you gave me an interesting feedback.

I would like to suggest to :

• add a contribution.md file
• mention the valid reasons you do not want to accept contributions for words, and what is the alternative
• mention about contribution md in the readme.md
• mention the contribution.md in words.go
• possibly create a pinned issue to mention you are not accepting contribution for words.go (and I'm fine with this)