e1000: add link to data sheet

Link to data sheet at intel.com so people can find it.
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>

hw/e1000.c

 /*
  * QEMU e1000 emulation
  *
+ * Software developer's manual:
+ * http://download.intel.com/design/network/manuals/8254x_GBe_SDM.pdf
+ *
  * Nir Peleg, Tutis Systems Ltd. for Qumranet Inc.
  * Copyright (c) 2008 Qumranet
  * Based on work done by: