#1240 Update readme with details on supply a custom HttpClient instance

[chowarth]

What kind of change does this PR introduce?

Readme update to include a description of supplying a custom HttpClient instance along with a potential pitfall and solution.

What is the current behavior?

#1240

What is the new behavior?

N/A

What might this PR break? N/A

Please check if the PR fulfills these requirements

• [ ] Tests for the changes have been added (for bug fixes / features)
• [x] Docs have been added / updated (for bug fixes / features)

Other information: