Provide better help / usage instructions

It was pointed out that the usage information provided by the application doesn't have clear enough instructions to get started, but the documentation in README.md does provide clear enough examples.

Enhance the `show_help` function in logpipe.c to include some examples of pipelines, and some background information describing the concepts behind it (e.g. the buffer, what steps are).

The end result is that after installing the package the user should be able to type `logpipe` and have all the information they need to start experimenting and get something useful out of it.