version to 2.6.0; rewrite docs for rolling release

This:

  - Changes version number everywhere I could find it.
  - Reworks get_version() to dispense with stable / dev division.
  - Rewrites CONTRIBUTING.md to describe a new release workflow.

The development/release workflow stuff could use some thought.  It's
clunky at best.  There's sort of an inherent tension between semver-style
version numbers and the rolling release thing - maybe we should just use
commit hash or something date-based?

Still, it's a start.

autoload/vimwiki/tags.vim

@@ -9,7 +9,8 @@
 
 " Tags metadata in-file format:
 "
-" Is based on CTags format (see |tags-file-format|).
+" Is based on CTags format (see |tags-file-format|) and
+" https://ctags.sourceforge.net/FORMAT
 "
 " {tagaddress} is set to lineno.  We'll let vim search by exact line number; we
 " can afford that, we assume metadata file is always updated before use.
@@ -291,8 +292,9 @@ function! s:write_tags_metadata(metadata) abort
     endfor
   endfor
   call sort(tags, 's:tags_entry_cmp')
+  " TODO: VimWiki version probably shouldn't be hardcoded here:
   let tag_comments = [
-    \ "!_TAG_PROGRAM_VERSION\t2.5",
+    \ "!_TAG_PROGRAM_VERSION\t2.6.0",
     \ "!_TAG_PROGRAM_URL\thttps://github.com/vimwiki/vimwiki",
     \ "!_TAG_PROGRAM_NAME\tVimwiki Tags",
     \ "!_TAG_PROGRAM_AUTHOR\tVimwiki",