feat(clients): update README with documentation, usage and more

[trivikr]

Issue #, if available:
Fixes: #1878
Fixes: #1905

Description of changes:
Updates client README with the following:

• Documentation of the client.
• Installation.
• Getting Started: Importing and Usage examples.
• Troubleshooting.
• Getting Help.
• Contributing.
• License.

Example README for DynamoDB Client

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov-io]

Codecov Report

Merging #1907 (1daf131) into master (f2a47e8) will increase coverage by 0.52%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1907      +/-   ##
==========================================
+ Coverage   79.30%   79.83%   +0.52%     
==========================================
  Files         368      368              
  Lines       15132    15549     +417     
  Branches     3222     3367     +145     
==========================================
+ Hits        12001    12413     +412     
- Misses       3131     3136       +5     

Impacted Files	Coverage Δ
packages/util-waiter/src/utils/validate.ts	81.81% <0.00%> (-18.19%)	⬇️
lib/storage/src/data-chunk/readable-helper.ts	86.84% <0.00%> (-5.47%)	⬇️
packages/s3-request-presigner/src/getSignedUrl.ts	92.85% <0.00%> (-2.98%)	⬇️
packages/util-dynamodb/src/convertToAttr.ts	98.70% <0.00%> (-1.30%)	⬇️
packages/util-waiter/src/waiter.ts	100.00% <0.00%> (ø)
protocol_tests/aws-ec2/endpoints.ts	81.48% <0.00%> (ø)
protocol_tests/aws-json/endpoints.ts	81.48% <0.00%> (ø)
protocol_tests/aws-query/endpoints.ts	81.48% <0.00%> (ø)
packages/util-dynamodb/src/marshall.ts	100.00% <0.00%> (ø)
protocol_tests/aws-restxml/endpoints.ts	81.48% <0.00%> (ø)
... and 141 more

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[aws-sdk-js-automation]

AWS CodeBuild CI Report

• CodeBuild project: sdk-staging-test
• Commit ID: 1daf131
• Result: SUCCEEDED
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[github-actions]

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