writerfilter: add a README explaining subdirs

writerfilter/README

+The writerfilter module contains import filters for Writer, using its
+UNO API.
+
+- inc: global headers (can be included by other modules)
+- prj: information for dmake
+- qa: unittests and subsequenttests
+- source: the filters themselves
+
+  * dmapper: the domain mapper, hiding UNO from the tokenizers
+  * doctok: incomplete ww8 (.doc) tokenizer
+  * filter: the uno filter service implementations
+  * ooxml: the docx tokenizer
+  * rtftok: the rtf tokenizer