Add list of client libraries to README

[SeanKilleen]

Description

Adds a list of currently existing client libraries to the README file for the overall Flagr project.

Motivation and Context

This makes it clear which languages might still benefit from implementation, and allows a central place to find client libraries even if they're not officially supported (a field could be added to denote official support later).

How Has This Been Tested?

Markdown preview within GitHub.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

None of the above. Minor docs change.

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov-commenter]

Codecov Report

Merging #383 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #383   +/-   ##
=======================================
  Coverage   83.01%   83.01%           
=======================================
  Files          27       27           
  Lines        1755     1755           
=======================================
  Hits         1457     1457           
  Misses        214      214           
  Partials       84       84           

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[zhouzhuojie]

Related to #378, I think we can add the list of clients (including all the community generated clients) to the docs - or even better, a dedicated page for SDK/clients and how to generate them from the openapi(swagger) spec.

https://github.com/checkr/flagr/blob/master/docs/_sidebar.md
https://checkr.github.io/flagr/#/