Adds LogHandler documentation to README

[abelsromero]

This PR adds a small documentation to explain how to register custom LogHandlers.

This is WIP and before merge I'd like to understand a couple of things I found in my tests in order to document them properly.

• paths in the Cursor are absolute always, I tried with relative Files and Strings. Is that expected?
• Some names are confusing to me: check the docs for the use of cursor.path, cursor.file and sourceFilename. Specially the latests. Is that also normal? If sourceFilename is always <script>, does it make sense to expose it?

[abelsromero]

Java 7 is failing, I wonder if due to lack of TSL 1.2 support.
The questions is, should we keep support for 7?

[robertpanzer]

Thanks for this awesome contribution!
With regards to the properties of the Cursor I am also unsure how to fully define them.

Regarding support for Java 7, I would like to keep at least that one for AsciidoctorJ 1.5.x even though the support from the CI seems to be lacking.
We already kicked out Java 6 with a minor release.
But I would be fine to disable the CI build for Java 7 if it is no longer working.

[abelsromero]

With regards to the properties of the Cursor I am also unsure how to fully define them.

OK then, if it's fine for you, we can merge.

I would like to keep at least that one for AsciidoctorJ 1.5.x

I can have a look at it. If the issue is what I think it's easy

[robertpanzer]

🙌 Thanks! 🎉