[CONTRUBITING] Add section on using a secondary devel directory

Add a section on how to use a separate directory than your normal
ZDOTDIR for development with its own zprezto.

CONTRIBUTING.md

@@ -33,6 +33,28 @@ there are a number of additional things to keep in mind.
    - Use the `my_func()` syntax for defining functions.
    - The 80 character hard limit can be waved for readability.
 
+#### Using an Alternative zprezto Directory
+
+To work on zprezto without messing with your current configuration:
+
+```sh
+mkdir devel-zprezto
+cd devel-zprezto
+git clone --recursive https://github.com/sorin-ionescu/prezto.git .zprezto
+ZDOTDIR=$(pwd)
+echo "Your development ZDOTDIR is $ZDOTDIR"
+setopt EXTENDED_GLOB
+for rcfile in "${ZDOTDIR:-$HOME}"/.zprezto/runcoms/^README.md(.N); do
+    ln -s "$rcfile" "${ZDOTDIR:-$HOME}/.${rcfile:t}"
+done
+```
+
+Then to start zsh in this development environment you will run:
+
+```sh
+ZDOTDIR=/path/to/devel-zprezto zsh
+```
+
 #### Modules
 
    - A *README.md* must be present.