Add unist-util-flat-filter to list of utilities

Closes GH-26.

Reviewed-by: Titus Wormer <[email protected]>

readme.md

@@ -446,6 +446,8 @@ unist:
  — Find nodes between two nodes or positions
 * [`unist-util-find-before`](https://github.com/syntax-tree/unist-util-find-before)
  — Find a node before another node
+* [`unist-util-flat-filter`](https://github.com/unicorn-utterances/unist-util-flat-filter)
+ — Create a new tree with all nodes that pass the given function without references to the parent
 * [`unist-util-flatmap`](https://gitlab.com/staltz/unist-util-flatmap)
  — Create a new tree by expanding a node into many
 * [`unist-util-generated`](https://github.com/syntax-tree/unist-util-generated)