Created Unicode and additional language support (markdown)

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+## Unicode support
+
+There are three Unicode keymap definition method available in QMK:
+
+### UNICODE_ENABLE
+
+Supports Unicode input up to 0xFFFF. The keycode function is `UC(n)` in
+keymap file, where *n* is a 4 digit hexadecimal.
+
+### UNICODEMAP_ENABLE
+
+Supports Unicode up to 0xFFFFFFFF. You need to maintain a separate mapping
+table `const uint32_t PROGMEM unicode_map[] = {...}` in your keymap file.
+The keycode function is `X(n)` where *n* is the array index of the mapping
+table.
+
+### UCIS_ENABLE
+
+TBD
+
+Unicode input in QMK works by inputing a sequence of characters to the OS,
+sort of like macro. Unfortunately, each OS has different ideas on how Unicode is inputted.
+
+This is the current list of Unicode input method in QMK:
+
+* UC_OSX: MacOS Unicode Hex Input support. Works only up to 0xFFFF. Disabled by default. To enable: go to System Preferences -> Keyboard -> Input Sources, and enable Unicode Hex.
+* UC_LNX: Unicode input method under Linux. Works up to 0xFFFFF. Should work almost anywhere on ibus enabled distros. Without ibus, this works under GTK apps, but rarely anywhere else.
+* UC_WIN: (not recommended) Windows built-in Unicode input. To enable: create registry key under `HKEY_CURRENT_USER\Control Panel\Input Method\EnableHexNumpad` of type `REG_SZ` called `EnableHexNumpad`, set its value to 1, and reboot. This method is not recommended because of reliability and compatibility issue, use WinCompose method below instead.
+* UC_WINC: Windows Unicode input using WinCompose. Requires [WinCompose](https://github.com/samhocevar/wincompose). Works reliably under many (all?) variations of Windows.
+
+## Additional language support
+
+In `quantum/keymap_extras/`, you'll see various language files - these work the same way as the alternative layout ones do. Most are defined by their two letter country/language code followed by an underscore and a 4-letter abbreviation of its name. `FR_UGRV` which will result in a `ù` when using a software-implemented AZERTY layout. It's currently difficult to send such characters in just the firmware.
+
+## International Characters on Windows
+
+[AutoHotkey](https://autohotkey.com) allows Windows users to create custom hotkeys among others.
+
+The method does not require Unicode support in the keyboard itself but depends instead of AutoHotkey running in the background.
+
+First you need to select a modifier combination that is not in use by any of your programs.
+CtrlAltWin is not used very widely and should therefore be perfect for this.
+There is a macro defined for a mod-tab combo `LCAG_T`.
+Add this mod-tab combo to a key on your keyboard, e.g.: `LCAG_T(KC_TAB)`.
+This makes the key behave like a tab key if pressed and released immediately but changes it to the modifier if used with another key.
+
+In the default script of AutoHotkey you can define custom hotkeys.
+
+    <^<!<#a::Send, ä
+    <^<!<#<+a::Send, Ä
+
+The hotkeys above are for the combination CtrlAltGui and CtrlAltGuiShift plus the letter a.
+AutoHotkey inserts the Text right of `Send, ` when this combination is pressed.
+