Update SignalFx receiver/exporter README

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[codecov]

Codecov Report

Merging #1009 into master will increase coverage by 13.89%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master    #1009       +/-   ##
===========================================
+ Coverage   74.88%   88.77%   +13.89%     
===========================================
  Files          21      247      +226     
  Lines        1071    13272    +12201     
===========================================
+ Hits          802    11782    +10980     
- Misses        213     1135      +922     
- Partials       56      355      +299     

Flag	Coverage Δ
#integration	74.88% <ø> (ø)
#unit	87.99% <ø> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
exporter/signalfxexporter/config.go	91.83% <ø> (ø)
...ceiver/carbonreceiver/protocol/plaintext_parser.go	93.54% <0.00%> (ø)
receiver/k8sclusterreceiver/utils/maps.go	100.00% <0.00%> (ø)
...rometheusexecreceiver/subprocessmanager/manager.go	74.13% <0.00%> (ø)
exporter/honeycombexporter/translator.go	88.88% <0.00%> (ø)
...iver/carbonreceiver/protocol/path_parser_helper.go	82.60% <0.00%> (ø)
...er/awsxrayreceiver/internal/translator/metadata.go	75.00% <0.00%> (ø)
receiver/splunkhecreceiver/factory.go	100.00% <0.00%> (ø)
receiver/carbonreceiver/transport/tcp_server.go	65.71% <0.00%> (ø)
...er/k8sclusterreceiver/collection/resourcequotas.go	100.00% <0.00%> (ø)
... and 229 more

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