docs(README.md): add Client usage info

[mr-linch]

close #33

[codecov]

Codecov Report

Merging #61 (848b9b7) into main (d107d97) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 848b9b7 differs from pull request most recent head 2f4b40c. Consider uploading reports for the commit 2f4b40c to get more accurate results

@@           Coverage Diff           @@
##             main      #61   +/-   ##
=======================================
  Coverage   90.06%   90.06%           
=======================================
  Files          17       17           
  Lines        1711     1711           
=======================================
  Hits         1541     1541           
  Misses        140      140           
  Partials       30       30           

Impacted Files	Coverage Δ
tgb/polling.go	84.07% <100.00%> (ø)
tgb/router.go	100.00% <100.00%> (ø)
tgb/webhook.go	85.71% <100.00%> (ø)

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