update PR template to ask about contributing issues

As discussed here[1], we should update the PR template to remind the author to add the contributing issue to the spec document if needed.

We will also need to define the criteria for this. For example, if the PR is just fixing typos or formatting then this is not necessary but if the PR is changing something about the spec then this is critical.

[1] https://github.com/awslabs/aws-encryption-sdk-specification/pull/74#discussion_r393369683

Start by reading the pull request template and the discussion linked in PR 74, then review how contributing issues are recorded in the specification document. Define when a PR must reference an issue in that document, update the template with the reminder and criteria, and verify that typo or formatting-only changes are exempt.

Written by the indexing model from the issue text.