docs(generic): add basic usage info

[piksel]

Adds some basic usage information that is missing from the generic service.

[codecov]

Codecov Report

Merging #254 (15562ab) into main (afa4eda) will increase coverage by 0.90%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #254      +/-   ##
==========================================
+ Coverage   76.95%   77.85%   +0.90%     
==========================================
  Files          85       96      +11     
  Lines        2703     2877     +174     
==========================================
+ Hits         2080     2240     +160     
- Misses        441      466      +25     
+ Partials      182      171      -11     

Impacted Files	Coverage Δ
pkg/util/partition_message.go	94.00% <0.00%> (-1.46%)	⬇️
pkg/services/discord/discord_json.go	90.47% <0.00%> (-1.20%)	⬇️
pkg/router/servicemap.go	100.00% <0.00%> (ø)
pkg/services/slack/slack_json.go	100.00% <0.00%> (ø)
pkg/services/matrix/matrix_client.go	70.68% <0.00%> (ø)
pkg/services/matrix/matrix_config.go	100.00% <0.00%> (ø)
pkg/services/telegram/telegram_config.go	100.00% <0.00%> (ø)
pkg/services/mattermost/mattermost_json.go	100.00% <0.00%> (ø)
pkg/services/bark/bark_config.go	100.00% <0.00%> (ø)
internal/testutils/textconfaker.go	0.00% <0.00%> (ø)
... and 21 more

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