[doc] added depends cross compile info

doc/build-unix.md

@@ -248,3 +248,26 @@ Additional Configure Flags
 A list of additional configure flags can be displayed with:
 
     ./configure --help
+
+
+
+ARM Cross-compilation
+-------------------
+These steps can be performed on, for example, an Ubuntu VM. The depends system
+will also work on other Linux distributions, however the commands for
+installing the toolchain will be different.
+
+First install the toolchain:
+
+    sudo apt-get install g++-arm-linux-gnueabihf
+
+To build executables for ARM:
+
+    cd depends
+    make HOST=arm-linux-gnueabihf NO_QT=1
+    cd ..
+    ./configure --prefix=$PWD/depends/arm-linux-gnueabihf --enable-glibc-back-compat --enable-reduce-exports LDFLAGS=-static-libstdc++
+    make
+
+
+For further documentation on the depends system see [README.md](../depends/README.md) in the depends directory.