add / update commands

[KazuCocoa]

This is a

• Bug report
• Question
• Freature Request

Summary

Appium repository starts to prepare documentation, and we'd love to check current methods and update them. Target is ruby_lib/lib/core.
https://github.com/appium/appium/labels/Documentation

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Good to start to add new commands

add https://github.com/appium/ruby_lib_core/issues?q=is%3Aissue+is%3Aopen+label%3A"good+first+issue"

[KazuCocoa]

1. Check commands in:
   • https://github.com/appium/ruby_lib/blob/master/lib/appium_lib/core/common/command.rb
   • https://github.com/appium/ruby_lib/search?utf8=✓&q=add_endpoint_method&type=
2. add tests
3. update or adding commands and create a pull-request to https://github.com/appium/appium/tree/master/commands-yml

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Add the way to adding new commands in contributing.md to be able to the others can update commands as well.

[KazuCocoa]

Add start/stop recording screen #674

[KazuCocoa]

• add script to get diff commands ruby_lib_core#9 (comment)
  • To get a list of which commands haven't been implemented

[KazuCocoa]

add https://github.com/appium/ruby_lib_core/issues?q=is%3Aissue+is%3Aopen+label%3A"good+first+issue"

[KazuCocoa]

update appium/doc

• add toggles ruby_lib_core#61
• Add some emulator commands ruby_lib_core#60