Add conditional job documentation

[lukasz-mitka]

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Please add documentation and examples for avoiding running jobs when comment is not in fact a 'slash command'.

if: startswith(github.event.comment.body, '/') appears to be a good way to do so. As in

jobs:
  slashCommandDispatch:
    if: startswith(github.event.comment.body, '/')

By avoiding running jobs it's possible to lower te bill on 'comment heavy' repos.