Construction
Before learning the "so what chord" it is better to know how fourth chords are built, so go take a look at this post about the quartal harmonization.
The "So what chord" is a five note chord voicing coming from the Dorian mode. It is built with three perfect fourths and one major third.
This creates a combination of quartal harmony (chords built in fourths) and tertiary harmony (chords built in thirds).
The example below show you how is built the Em11 chord. Starting from the degree I of the E Dorian mode, a first perfect fourth (A) is added to the root note (E), a second fourth (D) and a third fourth (G) are stacked.
Thus giving a perfect fourth chord in its pure form (3 fourths are stacked). Now, just add the major third (B) starting from the higher note of the fourth chord and you get the "So what chord" named Em7add11.
The "So what chord" is identical to the standard tuning of the five lowest strings of the guitar. (E-A-D-G-B). It is considered as m11 chord, but in theory m11 chords must contain a 9th.
That is not the case here, so what chords are made up of 1, 4, b7, b3 and 11. You can even name them m7add11.