Creates a layouts/ folder for keymaps shared between keyboards (#1609)

* include variables and .h files as pp directives

* start layout compilation

* split ergodoxes up

* don't compile all layouts for everything

* might seg fault

* reset layouts variable

* actually reset layouts

* include rules.mk instead

* remove includes from rules.mk

* update variable setting

* load visualizer from path

* adds some more examples

* adds more layouts

* more boards added

* more boards added

* adds documentation for layouts

* use lowercase names for LAYOUT_

* add layout.json files for each layout

* add community folder, default keymaps for layouts

* touch-up default layouts

* touch-up layouts, some keyboard rules.mk

* update documentation for layouts

* fix up serial/i2c switches

1 files changed, 77 insertions, 0 deletions

diff --git a/docs/feature_layouts.md b/docs/feature_layouts.md
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+++ b/docs/feature_layouts.md
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+# Layouts: Using a keymap with multiple keyboards
+
+The `layouts/` folder contains different physical key layouts that can apply to different keyboards. 
+
+```
+layouts/
++ default/
+| + 60_ansi/
+| | + readme.md
+| | + layout.json
+| | + a_good_keymap/
+| | | + keymap.c
+| | | + readme.md
+| | | + config.h
+| | | + rules.mk
+| | + <keymap folder>/
+| | + ...
+| + <layout folder>/
++ community/
+| + <layout folder>/
+| + ...
+```
+
+The `layouts/default/` and `layouts/community/` are two examples of layout "repositories" - currently `default` will contain all of the information concerning the layout, and one default keymap named `default_<layout>`, for users to use as a reference. `community` contains all of the community keymaps, with the eventual goal of being split-off into a separate repo for users to clone into `layouts/`. QMK searches through all folders in `layouts/`, so it's possible to have multiple reposistories here. 
+
+Each layout folder is named (`[a-z0-9_]`) after the physical aspects of the layout, in the most generic way possible, and contains a `readme.md` with the layout to be defined by the keyboard:
+
+```md
+# 60_ansi
+
+   LAYOUT_60_ansi
+```
+
+New names should try to stick to the standards set by existing layouts, and can be discussed in the PR/Issue.
+
+## Supporting a layout
+
+For a keyboard to support a layout, the variable (`[a-z0-9_]`) must be defined in it's `<keyboard>.h`, and match the number of arguments/keys (and preferrably the physical layout):
+
+    #define LAYOUT_60_ansi KEYMAP_ANSI
+
+The folder name must be added to the keyboard's `rules.mk`:
+
+    LAYOUTS = 60_ansi
+
+`LAYOUTS` can be appended in the subproject's `rules.mk`:
+
+    LAYOUTS += 60_iso
+
+but the `LAYOUT_<layout>` variable must be defined in `<subproject>.h` as well.
+
+## Tips for making layouts keyboard-agnostic
+
+Instead of using `#include "planck.h"`, you can use this line to include whatever `<keyboard>.h` (`<subproject>.h` should not be included here) file that is being compiled:
+
+    #include QMK_KEYBOARD_H
+
+In your config.h, you can also use this variable to include the keyboard's `config.h`:
+
+    #include QMK_KEYBOARD_CONFIG_H
+
+If you want to keep some keyboard-specific code, you can use these variables to escape it with an `#ifdef` statement:
+
+* `KEYBOARD_<keyboard>`
+* `SUBPROJECT_<subproject>`
+
+For example:
+
+```c
+#ifdef KEYBOARD_planck
+    #ifdef SUBPROJECT_rev4
+        planck_rev4_function();
+    #endif
+#endif
+```
+
+Note that the names are lowercase and match the folder/file names for the keyboard/subproject exactly.
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