Add to docs, replacing a layer package with a local version

When contributing to packages it is often handy to replace a package, already
installed and configured by some layer with a local version of the package (i.e.
a git repo). This commit adds instructions for how to achieve that.

doc/DOCUMENTATION.org

@@ -23,6 +23,7 @@
       - [[#declaration][Declaration]]
       - [[#initialization][Initialization]]
       - [[#exclusion][Exclusion]]
+      - [[#overriding-a-layer-package][Overriding a layer package]]
     - [[#without-a-layer][Without a layer]]
   - [[#packages-synchronization][Packages synchronization]]
   - [[#types-of-configuration-layers][Types of configuration layers]]
@@ -435,6 +436,9 @@ Example:
   (setq <layer>-excluded-packages '(package1 package2 ...)
 #+END_SRC
 
+**** Overriding a layer package
+     See [[https://www.spacemacs.org/doc/FAQ.html#how-to-override-a-layer-package][this answer in the FAQ's]].
+     
 *** Without a layer
 Sometimes a layer can be an unnecessary overhead, this is the case if you just
 want to install a package with very few configuration associated to it. A good

doc/FAQ.org

@@ -28,6 +28,7 @@
   - [[#i-want-to-learn-elisp-where-do-i-start-][I want to learn elisp, where do I start ?]]
 - [[#how-do-i][How do I]]
   - [[#install-a-package-not-provided-by-a-layer][Install a package not provided by a layer?]]
+  - [[#how-to-override-a-layer-package][How to override a layer package?]]
   - [[#disable-a-package-completely][Disable a package completely?]]
   - [[#disable-a-package-only-for-a-specific-major-mode][Disable a package only for a specific major-mode?]]
   - [[#disable-company-for-a-specific-major-mode][Disable company for a specific major-mode?]]
@@ -287,6 +288,27 @@ Spacemacs provides a variable in the =dotspacemacs/layers= function in
 to the list and it will be installed when you reload your configuration with
 ~SPC f e R~, or at the next Spacemacs launch.
 
+** How to override a layer package?
+   To replace a package that is installed and configured by a layer, without
+   losing the layer's configuration for that package, add the package to your
+ =dotspacemacs-additional-packages= with the =:location= keyword set to the
+   value =local=:
+  #+begin_src emacs-lisp :tangle yes
+    (package-name :location local)
+  #+end_src
+  The package should reside at =<layer>/local/<package>/= (this could be a
+  symbolic link to the real package (repo) directory). The package will still
+  get configured (but not loaded) by the layer. To load the package, require it
+  from your =dotspacemacs/user-config=: =(require 'package-name)=. In this way
+  you can easily switch between the custom version and the version configured by
+  the layer by commenting in/out the line in =dotspacemacs-additional-packages=
+  (to restore deferred loading also comment out the =require= form in
+ =dotspacemacs/user-config=).
+
+  You could also fully replace (i.e. overwrite) the layer version of the package
+  by using a Quelpa recipe with the pseudo-fetcher =local= as described [[https://develop.spacemacs.org/doc/LAYERS.html#packagesel][here]] and
+  [[https://github.com/quelpa/quelpa#file][here]].
+  
 ** Disable a package completely?
 To completely disable a package and effectively uninstalling it even if it is
 part of your used layers, look for the variable =dotspacemacs-excluded-packages=