DOC added KnownIssues document
DOC added KnownIssues document

Described issue about missing support for some characters set block names
in PCRE used for handling regular expressions in YANG modules.

PavolVican/libyang,lukasmacko/libyang,PavolVican/libyang,lukasmacko/libyang,PavolVican/libyang,PavolVican/libyang,PavolVican/libyang