Add monthly downloads badge to README

[Aman-Clement]

This PR is to add a monthly downloads count badge to the README.
There are more badges and widgets, you can view them here - https://piptrends.com/widgets/msgpack
(If necessary, the link from the badge to the package's pip Trends page can be removed. We just want to showcase a badge we have created.)

[ixmatus]

This person tried to open this exact PR against a project I'm the maintainer for as well (ixmatus/inflector#16). This is a promotional campaign and they (or colleagues) appear to be doing it on reddit too, I think this is suspicious activity. They are creating an external dependency on a web property that (as far as I can tell) is not officially blessed by pypi. I recommend rejecting this PR.

Users can get package statistics from libraries.io which is linked to directly by pypi itself.

[ayushjain01]

Thank you for your feedback and concerns. I'd like to clarify that this is not a promotional campaign. As stated in the PR description, our intention is simply to share a tool we have developed, which we believe could be beneficial to the community.

Our badge system offers highly accurate statistics, which is why we wanted to make it available. If the link to the pip trends page is a concern, we are more than willing to remove it.

We respect your decision if you prefer not to add more badges to your README. Our primary goal is to address any misunderstandings and assure you that this is a genuine effort to contribute to the open-source community by sharing our work on GitHub.

Thank you for considering our contribution.

[RaSan147]

funny enough 2 people trying to endorse the same PR also mass advertising, including SVG images in readme, potential of SVG XSS attack.