Add CONTRIBUTING.md

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Let's create documentation with guidelines for new contributors!

Any opinions would be appreciated.

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python/typeshed and pola-rs/polars are repositories that could be used as references.

For those contributors who would like to know about the appropriate amount of changes to submit in one PR, we should suggest google's Small CLs.

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I am doing "issue inventory" during my free time.
I thought there were cases where anyone could not react due to lack of written information about the environment or code snippets when an error occurred.

I found the microsoft/pylance-release issue format to be useful in getting information from the reporter.

I wrote the format based on that.

## Environment data

<!--
your `OS`, `Python` and `comtypes` version, what type libraries you want to use.
-->

## Code Snippet

<!--
Please provide a minimal, self-contained code snippet that reproduces the issue. GitHub permalink is very useful.
-->

# ```py
# XXX
# ```

## Situation reproducing Steps

<!--
Delete this section if not needed.
-->

1. XXX
2. XXX

## Expected behavior

XXX

## Actual behavior

XXX

Any opinions would be appreciated.