Documentation: Add documentation for Nios2 architecture

Signed-off-by: NLey Foon Tan <lftan@altera.com>

Documentation/nios2/README

+Linux on the Nios II architecture
+=================================
+
+This is a port of Linux to Nios II (nios2) processor.
+
+In order to compile for Nios II, you need a version of GCC with support for the generic
+system call ABI. Please see this link for more information on how compiling and booting
+software for the Nios II platform:
+http://www.rocketboards.org/foswiki/Documentation/NiosIILinuxUserManual
+
+For reference, please see the following link:
+http://www.altera.com/literature/lit-nio2.jsp
+
+What is Nios II?
+================
+Nios II is a 32-bit embedded-processor architecture designed specifically for the
+Altera family of FPGAs. In order to support Linux, Nios II needs to be configured
+with MMU and hardware multiplier enabled.
+
+Nios II ABI
+===========
+Please refer to chapter "Application Binary Interface" in Nios II Processor Reference
+Handbook.