docs: add beta notice

[prestonvasquez]

Add documentation noting that mongovector is using the 2.0.0-beta version of the Go Driver.

PR Checklist

• Read the Contributing documentation.
• Read the Code of conduct documentation.
• Name your Pull Request title clearly, concisely, and prefixed with the name of the primarily affected package you changed according to Good commit messages (such as memory: add interfaces for X, Y or util: add whizzbang helpers).
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. Fixes #123).
• Describes the source of new concepts.
• References existing implementations as appropriate.
• Contains test coverage for new functions.
• Passes all golangci-lint checks.

[prestonvasquez]

@tmc Here is an update to the docs file noting that mongovector uses the 2.0.0 beta. I'e created GODRIVER-3347 to track the upgrade to 2.0.0 stable.