When I was travelling on a road I was attacked by an assassin, I defeated him and he had a note

with my name on it and it said that someone had commissioned my murder, this is very strange

indeed and there will probably be more assassins, I wonder who I have pissed of that much?

Has anybody else experienced this?

There have been and are several assassin guilds in TES lore. The main two are the Morag Tong and it's rebelled offshoot The Dark Brotherhood.

The one you met is part of the random encounters feature. You didn't make anyone angry. (Well, your character may have at some point prior to starting the game.) It's automatic. They will show up for every character you make until you figure out how to stop them.

... or join them.

I can provide more detailed information about how to do that and whom to see. Alas, that will be by your direct inquiry as to prevent any loss of the rewarding element of discovery.

It doesn't sound like you have much to fear from these assassins. I generally fear such assassins at low levels - they are tough and persistent. My initial reaction is to run away, but at higher levels, to defeat and loot them. Unfortunately, I don't believe you can take their armor though they generally have gems, poisons and good weapons.

You can also get held up by a thief as you travel the wilds - they are also dangerous at low levels but generally have decent loot.