add simple contribution guide

[roeniss]

I had forked this repo because of #52 but the issue wasn't reproducible, so I quit.

And then, I thought it would be better to add some information to the document for contribution.

• what ruby version is required (3.1.x, 3.2.x doesn't work)
• how to run tests (maybe too basic, but I was lost a bit moment)
• how to make a package (same as above)