adds planck keymap readme

keyboard/planck/keymaps/README.md

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+# 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_user.h` *optional*, found automatically when compiling
+* `makefile.mk` *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 Planck keymaps
+
+* **default** default Planck layout