As per your need, you will not need to write code. Here is a hint:

1. Use workflow on Campaign instead and set evaluation criteria "Every time edited, subsequently meet criteria'

2. Set the condition in formula editor as ISCHANGED(Assigned_Resource__c). This condition will return true whenever the field changes and fire workflow action.

3. Set up workflow action "send email".

As per your need, you will not need to write code. Here is the approach:

1. Use workflow on Assigned_Resource__c instead and set evaluation criteria "Every time edited, subsequently meet criteria'

2. Set the condition in formula editor as ISCHANGED(Campaign Lookup field). This condition will return true whenever the field changes and fire workflow action.

3. Set up workflow action "send email".