If I fork a repo should I create a feature branch from master or develop ?

That would depend on why you fork?

Are you going to do pull requests back you should probably branch from dev.

Is the project dead and you wish to continue started work on dev also go from dev.

If dev is going in another direction or are dead and in bad form you might better of branching from master.

@Voxera thank you!

If there is no developer readme then do whatever you want

Who the fuck uses develop branches?