docs: add Playground configuration changes

[cbaker6]

New Pull Request Checklist

• I am not disclosing a vulnerability.
• I am creating this PR in reference to an issue.

Issue Description

Add information about how developers can connect the Swift Playgrounds to their own servers.

Related issue: #n/a

Approach

Update README and CONTRIBUTING

TODOs before merging

• Add changes to documentation (guides, repository pages, in-code descriptions)

[parse-github-assistant]

Thanks for opening this pull request!

• 🎉 We are excited about your hands-on contribution!

[codecov]

Codecov Report

Merging #312 (f1b6402) into main (e9ba846) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #312   +/-   ##
=======================================
  Coverage   84.58%   84.59%           
=======================================
  Files         114      114           
  Lines       11718    11718           
=======================================
+ Hits         9912     9913    +1     
+ Misses       1806     1805    -1     

Impacted Files	Coverage Δ
Sources/ParseSwift/Objects/ParseUser.swift	80.79% <0.00%> (+0.09%)	⬆️

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