docs: add semver label tag contributing guides and pull request todo (#…

…11084)

.github/PULL_REQUEST_TEMPLATE.md

@@ -25,5 +25,6 @@ the requirements below.
 - [ ] Tests are [changed or added](https://github.com/aws-amplify/amplify-cli/blob/dev/CONTRIBUTING.md#tests)
 - [ ] Relevant documentation is changed or added (and PR referenced)
 - [ ] New AWS SDK calls or CloudFormation actions have been added to relevant test and service IAM policies
+- [ ] [Pull request labels](https://github.com/aws-amplify/amplify-cli/blob/dev/CONTRIBUTING.md#labels) are added
 
 By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

CONTRIBUTING.md

@@ -104,10 +104,14 @@ Pull requests are welcome!
 
 You should open an issue to discuss your pull request, unless it's a trivial change. It's best to ensure that your proposed change would be accepted so that you don't waste your own time. If you would like to implement support for a significant feature that is not yet available, please talk to us beforehand to avoid any duplication of effort.
 
-Pull requests should generally be opened against **_dev_**.
+Pull requests should be opened against **_dev_**.
 
 Don't include any build files i.e. `dist/` in your PR. These will be built upon publish to npm and when a release is created on GitHub.
 
+### Labels
+
+If the change is a breaking change ([as defined by semantic versioning](https://semver.org/)), please add the `semver-major` label to your pull request on GiHub.
+
 ### Steps
 
 1. Go through the [Local Environment Setup](#local-environment-setup)