docs(exporter-collector): document breaking change in README

[blumamir]

Which problem is this PR solving?

• Following fix(exporter-collector): all http export requests should share same a… #1863, and due to the fact that two public attributes were removed from the node's collector exporter implementation

Short description of the changes

• Added a new section 0.15.0 to 0.16.0 under Upgrade guidelines in general README, which mentions this breaking change.

[codecov]

Codecov Report

Merging #1867 (f89e5d1) into main (072e38c) will increase coverage by 0.26%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1867      +/-   ##
==========================================
+ Coverage   92.25%   92.51%   +0.26%     
==========================================
  Files         159      174      +15     
  Lines        5189     6038     +849     
  Branches     1103     1283     +180     
==========================================
+ Hits         4787     5586     +799     
- Misses        402      452      +50     

Impacted Files	Coverage Δ
...async-hooks/src/AsyncLocalStorageContextManager.ts	25.00% <0.00%> (-75.00%)	⬇️
...kages/opentelemetry-node/src/NodeTracerProvider.ts	96.55% <0.00%> (-3.45%)	⬇️
...ry-exporter-collector/src/CollectorExporterBase.ts	92.45% <0.00%> (ø)
...kages/opentelemetry-web/src/StackContextManager.ts	97.05% <0.00%> (ø)
...s/opentelemetry-instrumentation-fetch/src/fetch.ts	99.23% <0.00%> (ø)
...ages/opentelemetry-exporter-collector/src/types.ts	100.00% <0.00%> (ø)
...emetry-instrumentation-xml-http-request/src/xhr.ts	96.82% <0.00%> (ø)
...mentation-xml-http-request/src/enums/EventNames.ts	100.00% <0.00%> (ø)
...lemetry-exporter-collector/src/transformMetrics.ts	95.00% <0.00%> (ø)
...ackages/opentelemetry-web/src/WebTracerProvider.ts	100.00% <0.00%> (ø)
... and 7 more