Adding CONTRIBUTING.md

[phyy-nx]

Draft of a CONTRIBUTING guide, based on conversation from the recent Telco. This guide tries to layout the existing NeXus contribution guidelines, including issue and pull request guides.

Feedback welcome.

[rayosborn]

LGTM although I would suggest capitalizing all the words in "NeXus international advisory committee."

[phyy-nx]

👍

[prjemian]

In the third paragraph, please change this text from:

describes a problem and answers "why" there needs to be work done

to:

describes a problem and answers "why" work needs to be done

[phyy-nx]

Done. I purport this is merely echoing the constitution and established practices and therefore doesn't need a vote, but disagreement is welcome.

[lukaspie]

In today's Telco, it was discussed that the preferred way of merging PR is doing a squash commit. This prevents the situation in #1424, where a couple hundreds a small commits were added to the history of the main branch. This had to be resolved in #1566 manually.

I think it would be a good idea to state this here explicitly (and also make squash commits the default in the settings, as @PeterC-DLS wanted to set it up).