Improve building from source instructions

[joshgoebel]

No description provided.

[asterite]

Thanks! But I don't want to make the Contributing file too big because maybe it makes it more tedious to read and encourages less contribution.

I recently updated the Contributing.md file to reference a list of dependencies. It also says that you need LLVM, so I think that's enough for now.

[asterite]

(I'm doing a pull-requests/issues cleanup, the number is starting to get higher)

[joshgoebel]

Is there harm in putting better directions in a separate file then if they don't belong here?

[asterite]

@yyyc514 Not at all, if you want we can create a Wiki page for that.