chore: update README for new milestones

[mayurkale22]

This updates the README to match the new milestones, and adds some new information about upcoming features.

[codecov-io]

Codecov Report

Merging #484 into master will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #484      +/-   ##
==========================================
+ Coverage   95.31%   95.35%   +0.04%     
==========================================
  Files         137      137              
  Lines        6973     7002      +29     
  Branches      596      596              
==========================================
+ Hits         6646     6677      +31     
+ Misses        327      325       -2

Impacted Files	Coverage Δ
...kages/opentelemetry-plugin-dns/test/utils/utils.ts	33.33% <0%> (-23.81%)	⬇️
.../opentelemetry-plugin-dns/test/utils/assertSpan.ts	100% <0%> (ø)	⬆️
...try-node/test/instrumentation/PluginLoader.test.ts	100% <0%> (ø)	⬆️
...try-plugin-dns/test/functionals/dns-enable.test.ts	100% <0%> (ø)	⬆️
...telemetry-core/src/context/propagation/B3Format.ts	96.15% <0%> (+0.15%)	⬆️
...core/src/context/propagation/BinaryTraceContext.ts	100% <0%> (+3.27%)	⬆️
...es/opentelemetry-node/src/instrumentation/utils.ts	96% <0%> (+5.52%)	⬆️