Update contributors guide and issue templates

[benmccann]

This adds the following:

• Suggest to ask maintainers about changes ahead of time
• Suggest that new methods be made private
• Suggest to add unit tests

Since a lot was added, I also wanted to try to keep it concise enough that everyone will read it, and so removed some information that was unnecessary or covered in another way:

• Remove the note to "Only change the individual files in /src". I'm not quite sure what this means. I believe it means don't check in the built files. We used to have these checked in prior to v2.2. E.g. https://github.com/chartjs/Chart.js/tree/v2.1.6/dist. We also want people to change tests, docs, and samples when appropriate, so I think it's clearer to remove this
• Combined the notes about running the linter and tests into a single line since gulp test covers both of these cases. There's a table down below that explains gulp lint, gulp unittest, etc. if users want to run only a subset of the checks
• Remove note about tabs vs spaces. This is covered by the line saying to run the linter

[simonbrunel]

I would certainly also add a note about not opening PR for major architecture changes, refactor, introduction of major features or API changes, without first getting approval on the expected API and/or implementation, either via creating a new ticket or via our slack #dev channel (not sure how to formulate).

I noticed that GitHub enhanced their issue creation workflow and I really like it (e.g. rollup). Maybe we could setup something similar (bug / enhancement / support)?

[benmccann]

Ok @simonbrunel. I added a note to ask about major changes before making them

I also like the idea of the issue types. I took a look at trying to set that up as well, but I believe that additional GitHub permissions are required, so you or @etimberg will need to do or grant someone else permission to manage that.

[simonbrunel]

I think we just need to create the associated files in the .github folder.

[benmccann]

Yeah, I think you're right. I just can't use the interactive editor on GitHub, but was able to create them manually. Take a look and let me know what you think.

[simonbrunel]

@benmccann I think we should be able to see it in action if you enable issues on your repository.

[benmccann]

I pushed it to my master branch @simonbrunel. Check it out here: https://github.com/benmccann/Chart.js/issues/new/choose

[simonbrunel]

https://github.com/benmccann/Chart.js/issues/new?template=FEATURE.md

It doesn't work for me, I see the fallback template (ISSUE_TEMPLATE.md)

[simonbrunel]

Looks really good @benmccann (I like the labels: 'type: *)

[simonbrunel]

Looks good, just a minor formatting change and I think we are good to go.

@benmccann Can you push changes as regular commits (no squash + force-push), it's easier to review changes :)

[etimberg]

Looks good. I'm happy to merge once the minor formatting issue @simonbrunel identified is fixed

[benmccann]

Ok. I've updated the formatting

[simonbrunel]

Thanks @benmccann