Improve documentation of available log types

[richardostler]

Just a quick note that 'farm_seeding' seems to be a valid log filter, although not listed in the readme documentation.

[mstenta]

Great observation @richardostler ! The README.md only lists the four basic log types (farm_activity, farm_observation, farm_input, and farm_harvest)...

But it also notes:

Other log types may be provided by add-on modules in farmOS.

That covers a number of other log types, but perhaps it would be worth improving the README.md a bit...

Maybe we could say something like:

These are the log types that come with farmOS (although may not be enabled depending on the modules that are turned on). Other log types may be provided by add-on modules in farmOS.

[mstenta]

Might also be worth pointing to the /farm.json API documentation: https://farmos.org/development/api/#farmjson

Each farmOS instance has a /farm.json endpoint that contains detailed information about the log types that are available on it. So any code that intends to work with farmOS generally should be checking that endpoint to make sure the log types it needs are available. That's not necessary for custom scripts that are only needed to work with a single farmOS instance - in that case you know what you have available.