mbed-os/.github/pull_request_template.md

### Description

<!-- 
    Required
    Add here detailed changes summary, testing results, dependencies 
    Good example: https://os.mbed.com/docs/latest/reference/workflow.html (Pull request template)
-->


### Pull request type

<!-- 
    Required
    Please add only one X to one of the following types. Do not fill multiple types (split the pull request otherwise).
    Please note this is not a GitHub task list, indenting the boxes or changing the format to add a '.' or '*' in front
    of them would change the meaning incorrectly. The only changes to be made are to add a description text under the
    description heading and to add a 'x' to the correct box.
-->
    [ ] Fix
    [ ] Refactor
    [ ] New target
    [ ] Feature
    [ ] Breaking change