Use ZDOTDIR env variable for path to .zshrc

.zshrc can be found at "~/.zshrc", when ZDOTDIR is not set.
If ZDOTDIR is set, .zshrc is in "${ZDOTDIR}/.zshrc".
Using parameter expansion, the hardcoded path can be replaced with
"${ZDOTDIR:-$HOME}/.zshrc". That way, it'll use the value in
ZDOTDIR when set and the value in HOME when not.
This thus mirrors the usage of ZDOTDIR in zsh and makes the code
more fexible.

README.md

@@ -29,7 +29,7 @@ features.
 ```bash
 git clone git://github.com/scmbreeze/scm_breeze.git ~/.scm_breeze
 ~/.scm_breeze/install.sh
-source ~/.bashrc   # or source ~/.zshrc
+source ~/.bashrc   # or source "${ZDOTDIR:-$HOME}/.zshrc"
 ```
 
 The install script creates required default configs and adds the following line
@@ -253,7 +253,7 @@ default base subdirectories are: Images, Backgrounds, Logos, Icons, Mockups,
 and Screenshots.
 
 After you have changed these settings, remember to run `source ~/.bashrc` or
-`source ~/.zshrc`.
+`source "${ZDOTDIR:-$HOME}/.zshrc"`.
 
 
 #### 2) Initialize design directories for your projects
@@ -324,7 +324,7 @@ Each feature is modular, so you are free to ignore the parts you don't want to
 use.  Just comment out the relevant line in `~/.scm_breeze/scm_breeze.sh`.
 
 **Note:** After changing any settings, you will need to run `source ~/.bashrc`
-(or `source ~/.zshrc`)
+(or `source "${ZDOTDIR:-$HOME}/.zshrc"`)
 
 I know we grow attached to the aliases we use every day, so I've made the alias
 system completely customizable.  You have two options when it comes to aliases: