CLI documentation

[Hipska]

Use Case

It would be nice to have a human readable documentation about the CLI command and all possible sub-commands + options in the repository. Just like we have a README.md for every plugin.

Expected behavior

Having a readme file in cmd/telegraf/ directory which not only says to run telegraf help but also has the actual documentation of the various commands and flags.

Actual behavior

Currently one needs to download/install telegraf binary and run its help command in order to read about a certain functionality.

Additional info

Could be autogenerated by running the make docs command. The CI could then warn the contributor if the repo is dirty and need to push the updated docs.

[srebhan]

Next steps: Implement a command to generate the readme from the help probably by running the binary or by cleverly extracting data from the command settings...