docs: add Playground configuration changes

[cbaker6]

New Pull Request Checklist

• [x] I am not disclosing a vulnerability.
• [x] 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

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

[parse-github-assistant[bot]]

Thanks for opening this pull request!

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

[codecov[bot]]

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%)	:arrow_up:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update e9ba846...f1b6402. Read the comment docs.