create base documentation (issue #59)

This version introduces multiple enhancements:

removed one unused documentation file, created whole content of package documentation, created some base description

Details of additions/deletions below:
--------------------------------------------------------------

Modified:   /docs/package.md
            -- Added --
                whole documentation

Modified:   /docs/errors.md
            -- Added --
                some base documentation

Modified:   /docs/how_it_works.md
            -- Updated --
                changed file name fomr how_it_works.php to how_it_works.md

Removed:    /docs/language-usage.md
            -- Removed --
                unused documentation file