Proposal : Add a Github PR template

[kwokcb]

Proposal

Add a Github PR template as part of the MaterialX repo.

• The default location seems to be: .github/PULL_REQUEST_TEMPLATE.md
• See docs

Possible properties / content:

## Summary
- What this is about

## Changes
- What was changed

## Tests
- If a fix / addition: specify testing done.

## Results
- Test or other results.

## Issue
update #<issue_number> | fixes #<issue number>