To me, it would seem more appropriate to wait until you start transition with, as you say, beginning with HRT. My reasoning is two-fold.
- If your preferred name and legal name are different, it will indicate that you are a trans person and that may be taken negatively.
- Also, what happens if the start of your HRT regimen is delayed for some reason? Progress should always be forward, if at all possible, and not back and forth.
If you are applying for a position where being LGBT is considered to be a positive, then such information may be helpful. Remember that a resume is not cast in concrete. Various versions might be needed to highlight some specific information that may be of value for particular employers.
Anyway, these are just thoughts. Think about what is appropriate for your particular situation.