can someone please tell me.....

Your question isn't stupid. I think even the most die-hard Bresson fan would appreciate the sensitivity of your question - seeing as a central theme of Bresson's film is the suffering of innocents, be they animals or humans.

As this remarkable film was made in 1966, seemingly before there was publicly expressed concern about cruelty to animals in film, it's not clear if the absence of a disclaimer about no animals being harmed is intentional or not. That is to say, whether a donkey was inevitably harmed (in which case a disclaimer's absence would be an honest omission) or if the donkey was not harmed but Bresson assumed his audience would understand that he had taken every precaution to insure the safety of the animal.
I am not an expert on what may or may not constitute cruelty to an animal during filming, but my feeling is that the violence inflicted upon Balthazar (the title donkey of the film) was filmed in such a way that it avoids cruelty to the donkey. Others may disagree with me. Here's a breakdown of the scenes of violence toward the donkey you see and some of my reasons for speculating that they don't constitute cruelty:

1) The donkey is shown being shoed, with the horseshoe heated and steam rising after its application to the hoof. I have no idea if horses and donkeys are still shoed this way, but in 1966 they probably still were. In which case, the film's depiction of it would be simply a documentary reenactment of a contemporary practice (one that perhaps today may be considered cruel, but in the context of the film's time period would have been considered normal and not cruel at all)

2) The donkey is shown being beaten at several (I believe 3 total) points in the film: once by a group of young men, another by a man with a stick, and towards the end again with a stick by one of the young men. I would argue (again, other viewers may disagree) that these blows are feinted, with the actors pulling their punches and blows in such a way that their full impact wasn't felt by the animal. Also, I do know that Bresson worked very meticulously with sound design on his films - so I'm almost positive that the sounds of the blows on the donkeys body were done in post-production foley recording. Anyone familiar with Bresson's work knows that he's not interested in a slavishly realistic portrayal of violence (for reference, consider the scene in which the band of young men kick the drunkard Arnold pretty lightly), but rather focuses on violence's spiritual ramifications.

3) In one rather difficult scene to watch, a young man gets the stubborn donkey to move by tying a piece of newspaper to its tail and setting it on fire. Though we briefly see the flame on the paper tied to the tail, Bresson almost immediately cuts to a wide shot in which the donkey runs away with the paper smoking (with no flame visible). So, my guess is that they lit the newspaper, the donkey ran out of frame (I'm not sure if it would have felt pain that close or not) and they then extinguished the flame. Somehow, the paper does produce a lot of smoke in that wide shot... not sure how they did that. Basically, my feeling (and hope) is that they were able to pull this off without harming the actual animal.

4) The donkey brays several times very loudly, on and off screen. But I don't think any violence was required to get these sounds.

At no time is any animal slaughtered in the film.

On a general note, somewhat related to your question, this film is not only a masterpiece in the history of cinema, but also an incredibly moving meditation on the lives (and sufferings) of animals.
The timing of your question seems to indicate you may be thinking of checking out the new Criterion edition of this film, which I saw last night. In an interesting interview with Bresson that appears in one of the extras, he insists that one of his main concerns was that the donkey remain the protagonist of the story throughout. Though this has many metaphysical implications, I believe that on a fundamental level Bresson did want to concentrate on the very real sufferings of an actual donkey. Given this, I believe that Bresson would have taken every possible precaution to see that the donkey used in the film was not harmed. Incidentally, one of the other extras on the DVD is an interview with film writer Donald Ritchie, in which he unfortunately, at one point, makes light of some complaints that the donkey may have been harmed during the shoot. Actually, it's not so clear cut - that is, not clear if Ritchie refers to Bresson literally hurting the donkey "playing" Balthazar or hurting (in terms of plot) the character of Balthazar... probably need to see it again.
Hope my response was not too lengthy or complicated. But this film is one of my favorite films of all time, one that I take very seriously. Your earnest question echoed similar thoughts I have had in the past - and I thought it deserved a careful, thorough response. I do hope you see and enjoy the film.