mbed-os/.github/pull_request_template.md

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### Description 
#### Summary of change <!-- Required -->

<!-- Basic information about the change: what the change is for, why do we need it any implications -->

#### Documentation <!-- Optional, but most likely you need it -->

<!-- Details of any document updates required, including links to PR against the docs repository -->

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### Pull request type <!-- Required -->

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    Required
    Please add only one X to one of the following types. Do not fill multiple types (split the pull request otherwise).
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    [] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
    [] Feature update (New feature / Functionality change / New API)
    [] Major update (Breaking change E.g. Return code change / API behaviour change)

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### Test results <!-- Required -->

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    Required
    Provide all the information required, listing all the testing performed. For new targets please attach full test results
    for all supported compilers.
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    [] No Tests required for this change (E.g docs only update)
    [] Covered by existing mbed-os tests (Greentea or Unittest)
    [] Tests / results supplied as part of this PR
    
    
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### Reviewers <!-- Optional -->

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    Optional
    Request additional reviewers with @username or @team
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### Release Notes <!-- Required for features, deprecations, breaking changes and other major PRs -->

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    All 3 sections are compulsory for Major PR types. For Feature PRs only the summary section is required.
    This section is automatically added to release notes. Please fill in each sub-section with sufficient detail for a user.
    For more information, please see [the contributing guidelines](https://os.mbed.com/docs/mbed-os/latest/contributing/workflow.html#pull-request-types). 
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#### Summary of changes

#### Impact of changes

#### Migration actions required