Add issue template for github issues

[Labreo]

Issue:
Currently sipnet is missing any kind of templates for issues unlike pecan.
Would be good for someone to implement it even though it is unnecessary,could act as a good first issue.

[dlebauer]

Good idea, like the PR template, the issue template should be based on and support the process documented in the CONTRIBUTING.md and related docs.

[ayushman1210]

Hey @dlebauer, is this issue still open? If no one is currently working on it, I’d be happy to take it up.

[Alomir]

I like PEcAn's triplet of bug report|feature request|doc change templates - we should do the same if we are going forward with this. Some comments about the PEcAn versions:

• bug report: we don't need machine info, though version is important
• doc changes: needs wordsmithing for sipnet, and the book editing link should be something like the Document section of CONTRIBUTING.md
• feature request: feature requests in sipnet should NOT require a proposed solution. In fact, I'd like to structure the template to encourage people to describe the problem without going into solution space. (Requirements before design before solution!)

[Alomir]

@ayushman1210 go for it!