Provide a home for transient files

README.markdown

@@ -31,6 +31,9 @@ mostly `:set` calls.)  Here's a taste:
 * `'incsearch'`: Start searching before pressing enter.
 * `'listchars'`: Show trailing whitespace.
 * `'scrolloff'`: Always show at least one line above/below the cursor.
+* `'directory'`: Dump swap files under `~/.cache/vim/swap` (only if that
+  directory already exists).  Also `'backupdir'` and `'undodir'`.  Try
+  `mkdir -p ~/.cache/vim/{swap,backup,undo}`.
 * `nmap Y y$`: Make `Y` behaves like `C` and `D`.
 * `runtime! macros/matchit.vim`: Load the version of matchit.vim that ships
   with Vim.