DOC: update contributing guide to reflect the preferred practice

[bsipocz]

Currently missing points:

• No outputs should be included in the notebooks
• we do prefer .md files as opposed to .ipynb.

Maybe the updated guide should be in a separate file, and keep the absolute newcomer version, that we currently have, in the README?

[bsipocz]

Maybe we can write this contributing guide in a generic way that can be reused elsewhere, too?

@rossbar: I suppose the preferred workflow is the same for the nx-guides?

[ankur0904]

@bsipocz I want to work on this issue.

[rossbar]

Thanks for your interest @ankur0904 - generally speaking this is probably not a great "first issue" as it deals mostly with documenting developer workflow as opposed to the tutorials themselves. If you're generally looking for ways to improve the tutorials, something like #152 would be great as it deals with improving user-facing tutorial content!

[ankur0904]

@rossbar Thanks for the suggestion. I must look into #152