Add NIU Mini from KBDFans (#1847)

* Add NIU Mini keymap from Planck keymap

* Remove old keymap files

* Fix README, removed Planck references

* Add default layout, move Planck layout to separate folder

* Update README

keyboards/niu_mini/keymaps/readme.md

@@ -0,0 +1,23 @@
+# How to add your own keymap
+
+Folders can be named however you'd like (will be approved upon merging), or should follow the format with a preceding `_`:
+
+    _[ISO 3166-1 alpha-2 code*]_[layout variant]_[layout name/author]
+
+\* See full list: https://en.wikipedia.org/wiki/ISO_3166-1#Officially_assigned_code_elements
+
+and contain the following files:
+
+* `keymap.c`
+* `readme.md` *recommended*
+* `config.h` *optional*, found automatically when compiling
+* `Makefile` *optional*, found automatically when compling
+
+When adding your keymap to this list, keep it organised alphabetically (select list, edit->sort lines), and use this format:
+
+     * **folder_name** description
+
+# List of keymaps
+
+- **default** default layout from KBDFans
+- **planck** Planck default layout