doc: Update README.md with supported OS/Arch

[andreaangiolillo]

Description

This PR updates the readme.md to add the supported os/arch

Link to any related issue(s): INTMDB-959

Type of change:

• Bug fix (non-breaking change which fixes an issue). Please, add the "bug" label to the PR.
• New feature (non-breaking change which adds functionality). Please, add the "enhancement" label to the PR.
• Breaking change (fix or feature that would cause existing functionality to not work as expected). Please, add the "breaking change" label to the PR.
• This change requires a documentation update
• Documentation fix/enhancement

Required Checklist:

• I have signed the MongoDB CLA
• I have read the contribution guidelines
• I have added tests that prove my fix is effective or that my feature works per HashiCorp requirements
• I have added any necessary documentation (if appropriate)
• I have run make fmt and formatted my code

Further comments

[colm-quinn]

LGTM, thanks for adding!

[gssbzn]

Can we also document this on the public/registry page?

[andreaangiolillo]

Can we also document this on the public/registry page?

Leave this question to @Zuhairahmed

[Zuhairahmed]

Can we also document this on the public/registry page?

Leave this question to @Zuhairahmed

No concerns, would add this same detail here: https://registry.terraform.io/providers/mongodb/mongodbatlas/latest/docs