gant: Added a caveat.

Added a caveat that notifies the user about a possible error if export GANT_HOME="/usr/local/opt/gant/libexec" is not entered into their .bashrc/.zshrc

The error the user would receive without entering export GANT_HOME="/usr/local/opt/gant/libexec" into their .bashrc/.zshrc was:
```
exception while configuring main class loader:
java.io.FileNotFoundException: /usr/local/Cellar/gant/1.9.10/conf/gant-starter.conf (No such file or directory)
	at java.io.FileInputStream.open(Native Method)
	at java.io.FileInputStream.<init>(FileInputStream.java:146)
	at java.io.FileInputStream.<init>(FileInputStream.java:101)
	at org.codehaus.groovy.tools.GroovyStarter.rootLoader(GroovyStarter.java:86)
	at org.codehaus.groovy.tools.GroovyStarter.main(GroovyStarter.java:128)
```
Be adding this caveat, users will now know how to avoid this error.