extend contributing.md to add PR guidelines

[iancrowther]

@simonmcmanus and I were saying that a PR should be complete and this should be reflected in the guide.

We should have a set of guides as to why a PR will be declined.

[Istar-Eldritch]

I would like to start taking some tasks related with the website. So far I've seen that we lack a CONTRIBUTING.md and that the contributing section in the readme is quite brief.

If @simonmcmanus is not working on this I would like to make a draft of such file so we can elaborate further from there.

[clarkie]

Sounds like a great idea!

[simonmcmanus]

that sounds great. Thanks Ruben! Let me know if you have any questions.

…

On 28 April 2017 at 13:52, Clarkie ***@***.***> wrote: Sounds like a great idea! — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#58 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AADaLcHcGUvbJYNuIx6KzBk0EpGdVdlLks5r0eD7gaJpZM4Gnoxk> .

-- Simon McManus DotJS Ltd - Node Consultancy