Update conference section with Expensify info

Why is this change being made?

Expensify does not contain a 'conference' category, and it is not obvious to new employees that conference tickets should be expensed as 'Employee Training.' This commit adds a sub-bullet explaining the correct category to use.

@nprecilla This MR is the result of my question to you earlier today.

Does this MR meet the acceptance criteria?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what