Added note about potential file name conflicts in Random.md

[matthamil]

In the Random exercise, it is not explicitly written that the reader should not name his/her file as Random.elm as a naming collision error will occur when he/she tries to run elm-make or elm-reactor.

See elm-lang/elm-make#104

[dskecse]

I reckon it may be the wrong place to add this info since the issue isn't directly related to the Random module.

[matthamil]

Where should this be added so others don't encounter this issue? @dskecse

If anything, a note could be added to this exercise page to inform users to not name their elm file random.elm.

[evancz]

I agree that people should be informed about this, but I do not think that this is an intuitive spot to add it. I'm not sure what to do instead.