docs: add reference to external plugin

[shiqizng]

Add a reference to CockroachDB exporter.

[codecov]

Codecov Report

Merging #143 (9f09cb3) into master (442791a) will increase coverage by 4.16%.
Report is 54 commits behind head on master.
The diff coverage is 81.00%.

@@            Coverage Diff             @@
##           master     #143      +/-   ##
==========================================
+ Coverage   67.66%   71.82%   +4.16%     
==========================================
  Files          32       37       +5     
  Lines        1976     2747     +771     
==========================================
+ Hits         1337     1973     +636     
- Misses        570      675     +105     
- Partials       69       99      +30     

Files Changed	Coverage Δ
conduit/data/block_export_data.go	100.00% <ø> (+92.30%)	⬆️
conduit/metrics/metrics.go	100.00% <ø> (ø)
conduit/pipeline/metadata.go	69.11% <ø> (ø)
conduit/plugins/config.go	100.00% <ø> (ø)
...duit/plugins/exporters/filewriter/file_exporter.go	81.63% <ø> (-1.06%)	⬇️
conduit/plugins/importers/algod/metrics.go	100.00% <ø> (ø)
...gins/processors/filterprocessor/fields/searcher.go	77.50% <ø> (ø)
...ins/processors/filterprocessor/filter_processor.go	83.82% <ø> (+3.54%)	⬆️
...plugins/processors/filterprocessor/gen/generate.go	34.28% <ø> (ø)
conduit/plugins/processors/noop/noop_processor.go	64.70% <ø> (+6.81%)	⬆️
... and 21 more

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