I would have thought it sensible to tell your employer - they have to make suitable arrangements to facilitate your work, eg allowing extra time, and if your dyslexia does cause problems, you will not have any recompense for lack of employer support if you have not informed them that you do have a disability. I have had to do an internal investiagtion with a previous employer regarding a claim of disability discrimination, and the claim that the employer had not taken appropriate steps; the claimant lost his claim as he had not at any point informed us as to his dyslexia, and other support had been given for his learning difficulties. I would suggest you be honest.