docs(test-utils): add README.md for @opentelemetry/test-utils

[rauno56]

Short description of the changes

As per our discussion during the SIG Meeting. I'm adding a readme for the package to be published.

#595

[codecov]

Codecov Report

Merging #596 (f4a2bc2) into main (75a3c87) will decrease coverage by 0.10%.
The diff coverage is n/a.

❗ Current head f4a2bc2 differs from pull request most recent head dffdbf5. Consider uploading reports for the commit dffdbf5 to get more accurate results

@@            Coverage Diff             @@
##             main     #596      +/-   ##
==========================================
- Coverage   94.94%   94.84%   -0.11%     
==========================================
  Files         195      182      -13     
  Lines       11637    11003     -634     
  Branches     1115      991     -124     
==========================================
- Hits        11049    10436     -613     
+ Misses        588      567      -21     

Impacted Files	Coverage Δ
...trumentation-document-load/src/enums/EventNames.ts
...entation-document-load/src/enums/AttributeNames.ts
...etry-plugin-react-load/src/enums/AttributeNames.ts
...ry-instrumentation-user-interaction/src/version.ts
...metry-instrumentation-document-load/src/version.ts
...strumentation-document-load/src/instrumentation.ts
...apackages/auto-instrumentations-web/src/version.ts
...ation-user-interaction/src/enums/AttributeNames.ts
...etapackages/auto-instrumentations-web/src/utils.ts
...umentation-user-interaction/src/instrumentation.ts
... and 3 more