What a great idea! Such a handout should be put in the pews as well as at the entrance to the church.
My advice: keep it short, short, short. Make it welcoming and non-threatening. Leave out the dress code. Visitors aren’t going to go home, change their clothes, and return. They’ll notice what others are wearing.
Let them know they’re free to do what everyone else does or to sit and observe, either is fine.
Do include the standard guidelines for communion, although they’re in the missalettes, visitors who aren’t familiar with the Catholic church may not see them there.
In some parishes, those who don’t receive communion may come up for a blessing (I don’t agree with this but that is beside the point). If your parish is one that does not, be sure the guidelines say what to do at communion time. For example, “For those who will not be receiving communion today, please remain in your seat.” Otherwise, visitors may come up for a blessing to the discomfort and/or confusion of the priest or EMHC. If your parish is one that does this, briefly explain the procedure. Those wishing a blessing should go to the priest, since the EMHCs have no authority to give a blessing.
It might be helpful to include where in the missalette to find the prayers of the mass.
My feeling is that this visitors’ handout should not contain an explanation of what the mass is. At the most, only one sentence. Let the mass speak for itself.
It would also be helpful to have little pamphlets available at the entrance of the church explaining what the mass is, the parts, and the history. Good ones can be purchased or the parish could print its own.