Update README.md - Add monthly downloads count badge

[ayushjain01]

Please answer these questions before submitting your pull requests. Thanks!

1. What GitHub issue is this PR addressing? Make sure that there is an accompanying issue to your PR.

   Fixes SNOW-1475363: Adding a monthly downloads badge to the README #502

2. Fill out the following pre-review checklist:

   • I am adding a new automated test(s) to verify correctness of my new code
   • I am adding new logging messages
   • I am adding new credentials
   • I am adding a new dependency

3. Please describe how your code solves the related issue.

Like mentioned in the issue, this PR is to add a monthly downloads count badge to the README file. This badge is from pip Trends. If needed the link to the package's pip Trends page can be removed from the badge. This is only to share the badge we have built. View more - https://piptrends.com/package/snowflake-sqlalchemy

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[ixmatus]

A colleague of 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.