Added Asciidoc contrib layer.

It has keybindings (very rudimentary, a lot more can be added later, see comments in source code).
It has :defer t
It has all its dependencies (cl mainly) properly defined
All necessary init functions are prepared
Moved to new `!lang` dir
Battle-tested by daily usage

contrib/!lang/asciidoc/README.md

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+# Asciidoc contribution layer for Spacemacs
+
+<!-- markdown-toc start - Don't edit this section. Run M-x markdown-toc/generate-toc again -->
+**Table of Contents**
+
+- [Asciidoc contribution layer for Spacemacs](#asciidoc-contribution-layer-for-spacemacs)
+    - [Description](#description)
+    - [Install](#install)
+    - [Key bindings](#key-bindings)
+        - [Element insertion](#element-insertion)
+        - [Promotion, Demotion](#promotion-demotion)
+
+<!-- markdown-toc end -->
+
+## Description
+
+This layer adds [AsciiDoc][] markup language support to Spacemacs.
+
+Feature
+- asciidoc format support via [adoc-mode][]
+- `.adoc` files are associated with `adoc-mode` by default
+
+## Install
+
+To use this contribution add it to your `~/.spacemacs`
+
+```elisp
+(setq-default dotspacemacs-configuration-layers '(asciidoc))
+```
+
+## Key bindings
+
+### Element insertion
+
+    Key Binding       |                 Description
+----------------------|------------------------------------------------------------
+<kbd>SPC m h 1<kbd>   | Insert title level 1
+<kbd>SPC m h I<kbd>   | Insert title level 1 (the most important one)
+<kbd>SPC m h 2<kbd>   | Insert title level 2
+<kbd>SPC m h i<kbd>   | Insert title level 2 (the most usual one)
+<kbd>SPC m h 3<kbd>   | Insert title level 3
+<kbd>SPC m h 4<kbd>   | Insert title level 4
+<kbd>SPC m h 5<kbd>   | Insert title level 5
+<kbd>SPC m h 5<kbd>   | Insert title level 5
+<kbd>SPC m x b<kbd>   | Boldface selected
+<kbd>SPC m x i<kbd>   | Italicize selected
+
+
+### Promotion, Demotion
+    Key Binding       |                 Description
+----------------------|------------------------------------------------------------
+<kbd>M-h<kbd>   | Promote title level
+<kbd>M-l<kbd>   | Denote title level
+
+[AsciiDoc]: https://asciidoctor.org
+[adoc-mode]: https://github.com/sensorflo/adoc-mode

contrib/!lang/asciidoc/packages.el

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+;;; packages.el --- Asciidoc Layer packages File for Spacemacs
+;;
+;; Copyright (c) 2012-2014 Sylvain Benner
+;; Copyright (c) 2015 Mark Safronov, Torben Hoffmann & Contributors
+;;
+;; Author: Mark Safronov <hijarian@gmail.com>
+;; Author: Torben Hoffmann <torben.lehoff@gmail.com>
+;; URL: https://github.com/syl20bnr/spacemacs
+;;
+;; This file is not part of GNU Emacs.
+;;
+;;; License: GPLv3
+
+(setq asciidoc-packages
+  '(cl adoc-mode))
+
+(defun asciidoc/init-adoc-mode ()
+  (use-package adoc-mode
+    :mode (("\\.adoc?$" . adoc-mode)) ; We will NOT default `.txt' files to AsciiDoc mode, and `.asciidoc' extension is just plain stupid.
+		:defer t
+    :config
+    (progn
+      ;; We have quite a lot of possible keybindings. See `adoc-mode.el', its bottom part where the huge easy-menu is defined and after that, where the various `tempo-template-*' functions are defined.
+
+      ;; See /doc/CONVENTIONS.md#plain-text-markup-languages
+      (evil-leader/set-key-for-mode 'adoc-mode
+        "mh1" 'tempo-template-adoc-title-1
+        "mhI" 'tempo-template-adoc-title-1 ; Alternative method of inserting top-level heading - capital "I"
+        "mh2" 'tempo-template-adoc-title-2
+        "mhi" 'tempo-template-adoc-title-2 ; Alternative method of inserting the most usual heading - lowercase "i"
+        "mh3" 'tempo-template-adoc-title-3
+        "mh4" 'tempo-template-adoc-title-4
+        "mh5" 'tempo-template-adoc-title-5
+        "mxb" 'tempo-template-adoc-strong
+        "mxi" 'tempo-template-adoc-emphasis)
+      (define-key adoc-mode-map (kbd "M-h") 'adoc-denote) ; yes, exactly like that. To "promote" title is to INCREASE its size. `adoc-denote' does the opposite: increases its LEVEL, which DECREASES its size.
+      (define-key adoc-mode-map (kbd "M-l") 'adoc-promote)))) ; see the comment about  adoc-denote above
+
+(defun asciidoc/init-cl ()
+  (use-package cl))
+