Feature/adding contributing.md

[eddelbuettel]

No description provided.

[kevinushey]

Looks great! Thanks for the minor tweaks.

[eddelbuettel]

So ... shall we fold it in? And thanks for your excellent start. I really only rounded out some corners and clarified a wee bit at the end.

[kevinushey]

Let's perhaps wait for one last look from @jjallaire and @thirdwing and then fold it in.

[eddelbuettel]

👍

[thirdwing]

👍

[eddelbuettel]

Thank you both. I will fold it now. Release 0.12.3 still on track and grinding through rev.dep. checks.