add FAQ for common setup problems

[roymarantz]

As part of https://github.com/tumblr/collins/issues/250 it was mentioned that

Typically the DbUtil class not found errors are because the APP_HOME is incorrect, so the classpath is not set appropriately (see https://github.com/tumblr/collins/blob/v1.3.0/scripts/collins.sh#L14 and https://github.com/tumblr/collins/blob/v1.3.0/scripts/collins.sh#L133 ). Have you overridden variables like APP_HOME to the root of your app's installation directory with /etc/sysconfig/collins? https://github.com/tumblr/collins/blob/v1.3.0/scripts/collins.sh#L35 sets the default which i suspect is not where your app is installed...

and not using bash for running collins scripts

are problems that happen often. They should be added to a FAQ for setup problems.