Creates a userspace for keymaps (#1559)

* create a user space

* adds example

* document, add readme.md

* jackhumbert userspace, ergodox keymap

docs/_summary.md

@@ -15,6 +15,7 @@
 * [Features](features.md)
   * [Layouts](feature_layouts.md)
   * [Common Shortcuts](feature_common_shortcuts.md)
+  * [Userspace](feature_userspace.md)
   * [Backlight](feature_backlight.md)
   * [Bootmagic](feature_bootmagic.md)
   * [Dynamic Macros](dynamic_macros.md)

docs/feature_userspace.md

@@ -0,0 +1,33 @@
+# Userspace: sharing code between keymaps
+
+If you use more than one keyboard with a similar keymap, you might see the benefit in being able to share code between them. Create your own folder in `users/` named the same as your keymap (ideally your github username, `<name>`) with the following structure:
+
+* `/users/<name>/` (added to the path automatically)
+  * `readme.md`
+  * `rules.mk` (included automatically)
+  * `<name>.h` (optional)
+  * `<name>.c` (optional)
+
+`<name>.c` will need to be added to the SRC in `rules.mk` like this:
+
+    SRC += <name>.c
+
+Additional files may be added in the same way - it's recommended you have one named `<name>`.c/.h though.
+
+All this only happens when you build a keymap named `<name>`, like this:
+
+    make planck:<name>
+
+For example, 
+
+    make planck:jack
+
+Will include the `/users/jack/` folder in the path, along with `/users/jack/rules.mk`.
+
+## Readme
+
+Please include authorship (your name, github username, email), and optionally [a license that's GPL compatible](https://www.gnu.org/licenses/license-list.html#GPLCompatibleLicenses).
+
+## Example
+
+For a brief example, checkout `/users/_example/` until we have more reasonable and useful examples.