Browser compatibility in docs

[rayshan]

Hello. Great library, thanks for all your work.

I see that the code uses array.map(), which isn't available for IE 8 and older. The docs does not state whether there's polyfill for IE. It would be helpful add a browser compatibility section in readme. Thanks.

[jcoglan]

Where's the reference to Array.map()? Sylvester dates from 2007 so was developed before this method was available, so it should not depend on it.

[rayshan]

I should have looked deeper. When I saw below in add I automatically thought array.map():
this.map(function(x, i, j) { return x + M[i-1][j-1]; })

But maybe it doesn't hurt to add a note about compatibility in readme?