WIP: Add Merge Request Template

Description

This MR adds an MR template for projects to include in their Merge Request workflows. Documentation is updated to reflect this.

This is tagged WIP for discussion purposes and once it is finalised it will be put to use in this project.

MR Template builds on @ben.mort's draft and the following references.

References:

• https://github.com/angular-translate/angular-translate/blob/master/.github/PULL_REQUEST_TEMPLATE.md
• https://github.com/appium/appium/blob/master/.github/PULL_REQUEST_TEMPLATE.md
• https://github.com/bchavez/RethinkDb.Driver/blob/master/.github/PULL_REQUEST_TEMPLATE.md
• https://github.com/ionic-team/ionic-framework/blob/master/.github/PULL_REQUEST_TEMPLATE.md
• https://github.com/go-sql-driver/mysql/blob/master/.github/PULL_REQUEST_TEMPLATE.md
• https://github.com/theos/theos/blob/master/.github/PULL_REQUEST_TEMPLATE.md
• https://github.com/Urigo/angular-meteor/blob/master/.github/PULL_REQUEST_TEMPLATE.md
• https://github.com/angular/angular.js/blob/master/DEVELOPERS.md#commits

Ideas/Questions to discuss:

• Type of Change could be included (I didn't add it to keep this simple as possible) (reply to comment)
• Definition of Done already includes different test-related entries, they could be removed but I left them to give them more emphasis (reply to comment)
• Add more in comments

Relevant tickets and issues

Checklist

• This code meets SKA Definition of Done
• This code conforms to the SKA Coding Standards (see Python, Javascript, VHDL, C++)
• Unit tests has been added/updated to cover all changes and the resulting test coverage has not decreased
• Component and/or interface tests have been added for any new externally facing APIs (preferably using a BDD style)
• All end-to-end system tests pass when run in a recognised SKA integration environment
• All related tests follow SKA Testing Policy and Strategy
• Project's pipeline is not broken
• This code has been documented according to SKA Documentation Guidelines
• Relevant user documentation has been updated
• Commit messages follow SKA Commit Guidelines