Support README in playbooks

[chungers]

If a README.md file is included in a directory of a playbook, automatically add it in the help text of the corresponding command.

Signed-off-by: David Chung david.chung@docker.com

[codecov]

Codecov Report

Merging #471 into master will decrease coverage by 0.04%.
The diff coverage is 25%.

@@            Coverage Diff             @@
##           master     #471      +/-   ##
==========================================
- Coverage   68.23%   68.18%   -0.05%     
==========================================
  Files          69       69              
  Lines        4155     4159       +4     
==========================================
+ Hits         2835     2836       +1     
- Misses       1065     1067       +2     
- Partials      255      256       +1

Impacted Files	Coverage Δ
pkg/cli/remote/remote.go	60% <25%> (-1.98%)	⬇️

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