Update docs to reflect status of support for services

[jalevin]

Helps notify others that services are not currently supported #173

[codecov]

Codecov Report

Merging #1180 (7d475ca) into master (4f8da0a) will increase coverage by 4.71%.
The diff coverage is 78.44%.

@@            Coverage Diff             @@
##           master    #1180      +/-   ##
==========================================
+ Coverage   57.50%   62.22%   +4.71%     
==========================================
  Files          32       40       +8     
  Lines        4594     5321     +727     
==========================================
+ Hits         2642     3311     +669     
- Misses       1729     1744      +15     
- Partials      223      266      +43     

Impacted Files	Coverage Δ
pkg/model/action.go	0.00% <0.00%> (ø)
pkg/model/github_context.go	79.51% <ø> (ø)
pkg/model/planner.go	50.73% <ø> (+0.32%)	⬆️
pkg/model/step_result.go	0.00% <ø> (ø)
pkg/model/workflow.go	54.29% <0.00%> (+3.38%)	⬆️
pkg/container/docker_run.go	15.00% <26.82%> (+9.45%)	⬆️
pkg/common/git.go	49.31% <28.57%> (-0.52%)	⬇️
pkg/container/file_collector.go	44.85% <44.85%> (ø)
pkg/exprparser/interpreter.go	73.37% <53.48%> (-0.02%)	⬇️
pkg/runner/runner.go	70.16% <66.66%> (-6.31%)	⬇️
... and 24 more

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