Add instructions on how to use .tree and .errors files

[scadgek]

Looking through the sources, I suppose it's possible to specify the expected tree outputs and expected errors for some test files. It would be very useful to have a documentation with examples describing how to use that files.
Also, when some of the test files contains an error (does not match grammar syntax), the plugin tries to open .errors file and throws an error if it doesn't exist:

Caused by: java.io.FileNotFoundException: /home/scadge/IdeaProjects/querysyntax/query-syntax/examples/dml_select.txt.errors (No such file or directory)

Though it's probably a subject to another issue, I thought it would be useful to also mention it here.