I know how to modify interfaces file to enable raspberry pi work under WIFI.
But for raspberry pi 2, installed of snappy ubuntu, the file is on a read-only file system and I cannot modify it. Like below:

    ubuntu@localhost:/etc/network$ cat interfaces
    # interfaces(5) file used by ifup(8) and ifdown(8)
    # Include files from /etc/network/interfaces.d:
    source-directory /etc/network/interfaces.d
    ubuntu@localhost:/etc/network$ touch 1
    touch: cannot touch ?.?. Read-only file system

Without modifying it, I cannot enable the USB WIFI dongle. How to solve it?