feat: add download badge in readme

[WilliamBergamin]

Summary

The goal of this PR is to add the a downloads badge to the readme, the title and badges were also centered.

Testing

look at this fork to see the rendered version

Category (place an x in each of the [ ])

• slack_sdk.web.WebClient (sync/async) (Web API client)
• slack_sdk.webhook.WebhookClient (sync/async) (Incoming Webhook, response_url sender)
• slack_sdk.socket_mode (Socket Mode client)
• slack_sdk.signature (Request Signature Verifier)
• slack_sdk.oauth (OAuth Flow Utilities)
• slack_sdk.models (UI component builders)
• slack_sdk.scim (SCIM API client)
• slack_sdk.audit_logs (Audit Logs API client)
• slack_sdk.rtm_v2 (RTM client)
• /docs-src (Documents, have you run ./scripts/docs.sh?)
• /docs-src-v2 (Documents, have you run ./scripts/docs-v2.sh?)
• /tutorial (PythOnBoardingBot tutorial)
• tests/integration_tests (Automated tests for this library)
• README.md

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've run python3 -m venv .venv && source .venv/bin/activate && ./scripts/run_validation.sh after making the changes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (03bd99e) 85.65% compared to head (df17867) 85.66%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1449   +/-   ##
=======================================
  Coverage   85.65%   85.66%           
=======================================
  Files         111      111           
  Lines       12137    12137           
=======================================
+ Hits        10396    10397    +1     
+ Misses       1741     1740    -1     

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[srajiang]

I noticed the contact/support badge is not visible in the badges rendered on your fork. Assuming that's intentional?

[seratch]

I believe it should be intentional. If so, I am fine with it too as the README mentions the GH issue tracker and community links at the bottom.

[seratch]

When the convo about the support link is resolved, you can merge this PR!

[WilliamBergamin]

@srajiang I don't think the contact/support badge was still relevant, I tested it out and the link redirected me to one of my workspaces /help/requests which seems to be targeted for general slack users not developer specific