Implement Issue and PR templates

As suggested by @brechmos-stsci , it might be beneficial to give users a check list when they open a new issue or a new pull request (the latter should be consistent with existing dev guide). A first draft could be obtained from existing sources, for example https://www.talater.com/open-source-templates .

For instance, choosing new issue, bug + feature request, and backend brought me to https://www.talater.com/open-source-templates/#/page/98

<!--- Provide a general summary of the issue in the Title above -->

## Expected Behavior
<!--- If you're describing a bug, tell us what should happen -->
<!--- If you're suggesting a change/improvement, tell us how it should work -->

## Current Behavior
<!--- If describing a bug, tell us what happens instead of the expected behavior -->
<!--- If suggesting a change/improvement, explain the difference from current behavior -->

## Possible Solution
<!--- Not obligatory, but suggest a fix/reason for the bug, -->
<!--- or ideas how to implement the addition or change -->

## Steps to Reproduce (for bugs)
<!--- Provide a link to a live example, or an unambiguous set of steps to -->
<!--- reproduce this bug. Include code to reproduce, if relevant -->
1.
2.
3.
4.

## Context
<!--- How has this issue affected you? What are you trying to accomplish? -->
<!--- Providing context helps us come up with a solution that is most useful in the real world -->

## Your Environment
<!--- Include as many relevant details about the environment you experienced the bug in -->
* Version used:
* Environment name and version (e.g. PHP 5.4 on nginx 1.9.1):
* Server type and version:
* Operating System and version:
* Link to your project:

[agarwalrounak]

@pllim I don’t think that only admins can make those templates. All we have to do is create files named ISSUE_TEMPLATE.md and PULL_REQUEST_TEMPLATE.md, and store them in .github folder but as @bsipocz mentioned, I should perhaps solve a good first issue and then move to this one.

[MSeifert04]

I think what @mhvk means is what we also have in ccdproc as file Templates (https://github.com/astropy/ccdproc/tree/master/.github).

Official github docs about them: https://help.github.com/articles/manually-creating-a-single-issue-template-for-your-repository/