x86 Assembly: Data in the Text Section

I don't quite understand how variables can be stored in the text section and how they can be manipulated. Shouldn't all variables be in the .data section and aren't all part of the .text section read-only? How does this code work then?

[Code taken from Shellcoder's Handbook]

Section .text
global _start

_start:
    jmp short GotoCall

shellcode:
    pop esi
    xor eax, eax
    mov byte [esi + 7], al
    lea ebx, [esi]
    mov long [esi + 8], ebx
    mov long [esi + 12], eax
    mov byte al, 0x0b
    mov ebx, esi
    lea ecx, [esi + 8]
    lea edx, [esi + 12]
    int 0x80

GotoCall:
    call shellcode
    db '/bin/shJAAAAKKKK'