Add LoggerFactory docs

[danicheg]

These docs are based on work made by @lorandszakacs in #421. So all credits go to them.
This covers a little part of the LoggerFactory, a basic functionality at most. It'd be great to complete it subsequently.

[danicheg]

I will be pretty happy if @hamnis and @lorandszakacs can have to look at this.

[hamnis]

Great stuff!

[FunFunFine]

Suggested change

	* you have control of loggers creation, and you can even add in whatever custom
	functionality you need for your applications here. E.g. create loggers that also push logs
	to some external providers by giving a custom implementation of this trait.
	* you have control of loggers creation, and you can even add in whatever custom
	functionality you need for your applications here by giving a custom implementation of this trait. For example, create loggers that also push logs to some external providers.

describe the way before talking about examples

[lorandszakacs]

Looks good, I'd incorporate @FunFunFine's suggestions though

[danicheg]

I've tackled most of the comments, so probably this PR is quite ok to get merged. If anyone wants to continue improving these docs, don't be shy and open a PR.