imp: Rename dependencies command and add docs

[Esemesek]

Summary

Renamed the dependencies command to get-dependencies and added documentation. cc @cpojer

Test plan

Running metro in command line and compiled documentation markdown.

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[codecov-io]

Codecov Report

Merging #339 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #339   +/-   ##
=======================================
  Coverage   85.06%   85.06%           
=======================================
  Files         172      172           
  Lines        5221     5221           
  Branches      797      797           
=======================================
  Hits         4441     4441           
  Misses        693      693           
  Partials       87       87

Impacted Files	Coverage Δ
packages/metro/src/commands/dependencies.js	23.33% <ø> (ø)	⬆️

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Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Facebook open source project. Thanks!

[facebook-github-bot]

@cpojer is landing this pull request. If you are a Facebook employee, you can view this diff on Phabricator.

[cpojer]

That's great, thank you! You should now be able to get rid of this code in the react-native-cli code: https://github.com/react-native-community/react-native-cli/blob/master/packages/local-cli/dependencies/dependencies.js