Remove deprecated information from langreadme

As per earlier commit e593fed9cecc948915c26ec6975d00181d86d18a we no
longer have any non-UTF8 language files. langreadme.txt has been updated
to remove reference to these old non-UTF8 files. References to the
translatewiki.net Mantis project and introduction guide have been added
to assist users who may be interested in helping translation efforts.