I've personally never found much benefit in outing myself (especially to strangers) months before transition, it just leaves people confused.
If you're starting HRT it will take some time before changes become evident, probably six months. Depending on what you have to wear and how well you can conceal breast development would mean no one would suspect anything for up to nine months or more. If you feel like going full time at that point it would be a case of outing yourself to the boss then, you would have proven yourself in the job so you would be better placed to transition then rather than announcing yourself as trans* from the outset.