Add information about `global-git-commit-mode`

Fix #2522

doc/FAQ.org

@@ -30,6 +30,7 @@
    - [[Restore the sentence delimiter to two spaces?][Restore the sentence delimiter to two spaces?]]
    - [[Prevent the visual selection overriding my system clipboard?][Prevent the visual selection overriding my system clipboard?]]
    - [[Make spell-checking support curly quotes (or any other character)?][Make spell-checking support curly quotes (or any other character)?]]
+   - [[Use Spacemacs as the =$EDITOR= for git commits?][Use Spacemacs as the =$EDITOR= for git commits?]]
  - [[Windows][Windows]]
    - [[Why do the fonts look crappy on Windows?][Why do the fonts look crappy on Windows?]]
    - [[Why is there no Spacemacs logo in the startup buffer?][Why is there no Spacemacs logo in the startup buffer?]]
@@ -354,6 +355,15 @@ add a symbol within =['’]=) to make it supported. Consult the help of
 You finally have to set =my_english= as your =ispell-local-dictionary= in order
 to use the dictionary supporting your newly added characters.
 
+** Use Spacemacs as the =$EDITOR= for git commits?
+Spacemacs can be used as the =$EDITOR= (or =$GIT_EDITOR=) for editing git
+commits messages. To enable this you have to add the following line to your
+=dotspacemacs/user-config=:
+
+#+begin_src emacs-lisp
+(global-git-commit-mode t)
+#+end_src
+
 * Windows
 ** Why do the fonts look crappy on Windows?
 You can install [[https://code.google.com/p/mactype/][MacType]] on Windows to get very nice looking fonts. It is

layers/+source-control/git/README.org

@@ -12,6 +12,7 @@
    - [[Magit status fullscreen][Magit status fullscreen]]
    - [[Magit auto-complete][Magit auto-complete]]
    - [[Magit SVN plugin][Magit SVN plugin]]
+   - [[Global git commit mode][Global git commit mode]]
    - [[Git][Git]]
    - [[Git-Flow][Git-Flow]]
    - [[Org integration][Org integration]]
@@ -78,6 +79,15 @@ layer by setting the variable =git-enable-magit-svn-plugin= to =t=.
   )
 #+END_SRC
 
+** Global git commit mode
+Spacemacs can be used as the =$EDITOR= (or =$GIT_EDITOR=) for editing git
+commits messages. To enable this you have to add the following line to your
+=dotspacemacs/user-config=:
+
+#+begin_src emacs-lisp
+(global-git-commit-mode t)
+#+end_src
+
 ** Git
 Of course if your OS does not ship with git (!) you'll have to install it
 on your machine. You can download it from the [[http://git-scm.com/downloads][download page]].