By default every snippets directory in runtimepath will be searched for snippets.
See :h rtp and nvim-snippy
snippets/<filetype>.snippets file will be autoloaded when the filetype matched.
snippets/_.snippets will be autoloaded for all filetypes.
The snippet syntax follows SnipMate.
Other details see https://github.com/dcampos/nvim-snippy#adding-snippets .