Add exporter guide

[mayurkale22]

Which problem is this PR solving?

• OpenTelemetry exporters can be contributed by anyone, this guide will provide a means for developers/vendors/users to create their own.

[codecov-io]

Codecov Report

Merging #575 into master will decrease coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #575      +/-   ##
==========================================
- Coverage   92.53%   92.46%   -0.08%     
==========================================
  Files         158      155       -3     
  Lines        7999     7868     -131     
  Branches      684      683       -1     
==========================================
- Hits         7402     7275     -127     
+ Misses        597      593       -4

Impacted Files	Coverage Δ
...ges/opentelemetry-exporter-zipkin/src/transform.ts	100% <0%> (ø)	⬆️
.../opentelemetry-exporter-zipkin/test/zipkin.test.ts	100% <0%> (ø)	⬆️
...try-node/test/instrumentation/PluginLoader.test.ts	100% <0%> (ø)	⬆️
...telemetry-scope-base/test/NoopScopeManager.test.ts	100% <0%> (ø)	⬆️
...entelemetry-exporter-zipkin/test/transform.test.ts	100% <0%> (ø)	⬆️
packages/opentelemetry-metrics/src/types.ts
...ackages/opentelemetry-exporter-zipkin/src/utils.ts
packages/opentelemetry-node/src/config.ts
...es/opentelemetry-node/src/instrumentation/utils.ts	100% <0%> (+9.52%)	⬆️
...kages/opentelemetry-metrics/test/mocks/Exporter.ts	100% <0%> (+33.33%)	⬆️

[obecny]

lgtm

[OlivierAlbertini]

Nice!