chore(docs:postgres): add usage instructions

[markwolff]

Which problem is this PR solving?

• Requires examples: provide an example for postgres plugin #491 & feat(plugin): pg-pool plugin implementation #501
• Closes Add usage instruction for postgres (pg) plugin #518

Short description of the changes

• Adds usage instructions for pg plugin. Since the pg module has a dependency on the pg-pool package, these instructions also include pg-pool references/links. These links are broken until pg-pool stuff gets merged in.

[codecov-io]

Codecov Report

Merging #539 into master will decrease coverage by 2.16%.
The diff coverage is 90.38%.

@@            Coverage Diff             @@
##           master     #539      +/-   ##
==========================================
- Coverage   91.91%   89.74%   -2.17%     
==========================================
  Files         169      192      +23     
  Lines        8249    10095    +1846     
  Branches      726      911     +185     
==========================================
+ Hits         7582     9060    +1478     
- Misses        667     1035     +368

Impacted Files	Coverage Δ
...ackages/opentelemetry-exporter-jaeger/src/types.ts	100% <ø> (ø)	⬆️
...pe-async-hooks/test/AsyncHooksScopeManager.test.ts	100% <ø> (ø)	⬆️
packages/opentelemetry-core/src/common/types.ts	100% <ø> (ø)	⬆️
...ntelemetry-web/src/enums/PerformanceTimingNames.ts	100% <ø> (ø)
packages/opentelemetry-node/src/config.ts	100% <ø> (ø)
...ges/opentelemetry-exporter-jaeger/src/transform.ts	100% <100%> (ø)	⬆️
...entelemetry-exporter-jaeger/test/transform.test.ts	100% <100%> (ø)	⬆️
...lugin-https/test/functionals/https-disable.test.ts	100% <100%> (ø)
...emetry-plugin-xml-http-request/src/enums/Format.ts	100% <100%> (ø)
packages/opentelemetry-plugin-ioredis/src/enums.ts	100% <100%> (ø)
... and 105 more

[xiao-lix]

Is it necessary to add supported version of pg-pool here?

• pg-pool: 2.x

[markwolff]

Good call. Added in 0af2c10