fix: move pull request template (sashabaranov#420)

Author: vvatanabe

.github/PULL_REQUEST_TEMPLATE/pull_request_template.md renamed to .github/PULL_REQUEST_TEMPLATE.md

@@ -10,6 +10,8 @@ assignees: ''
 A similar PR may already be submitted!
 Please search among the [Pull request](https://github.com/sashabaranov/go-openai/pulls) before creating one.
 
+If your changes introduce breaking changes, please prefix the title of your pull request with "[BREAKING_CHANGES]". This allows for clear identification of such changes in the 'What's Changed' section on the release page, making it developer-friendly.
+
 Thanks for submitting a pull request! Please provide enough information so that others can review your pull request.
 
 **Describe the change**