Add GitHub pull request template

[scop]

Hm, we should add a template for pull requests so that the request to ask upstream to include the completion first is more prominent. Anyway, for time being, see https://raw.githubusercontent.com/scop/bash-completion/master/.github/ISSUE_TEMPLATE/feature_request.md and the start of CONTRIBUTING.md.

Originally posted by @scop in #639 (comment)