write .filetags-tagtree and .filetags-tagfilter file to their root directories

[novoid]

When filetags is generating TagTrees or tagfilter hierarchies, there should be .filetags-tagtree and .filetags-tagfilter files in their root directories so that users are able to locate them for purposes like, e.g., excluding from backup runs.

Those files should contain explanations on filetags, TagTrees/tagfilter so that people are able to learn about it.