chore: add a pull request template

[vvatanabe]

@sashabaranov I've created a pull request template for bug reports. Could you please review the content of the template?

[codecov]

Codecov Report

Merging #412 (50faedf) into master (581f70b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #412   +/-   ##
=======================================
  Coverage   96.95%   96.95%           
=======================================
  Files          17       17           
  Lines         690      690           
=======================================
  Hits          669      669           
  Misses         15       15           
  Partials        6        6           

[vvatanabe]

@sashabaranov
How about prefixing the title of pull requests that contain breaking changes with [BREAKING_CHANGES]? This would make it immediately clear on the release page under the 'What's Changed' section that a breaking change has been introduced. (I presume the release page is auto-generated by a tool, right?) This approach is very developer-friendly. If there are no objections, I want to incorporate that rule into this pull request template.

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Here's an example (Note that no actual changes have been made).

[sashabaranov]

Thank you so much! I agree with the [BREAKING_CHANGES] suggestion, it would definitely help!