add cross compile docs

CROSSCOMPILE.md

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+# Cross Compile
+
+Currently supported targets:
+
+Tier 1:
+
+* Linux (arm/x86)
+* Windows (32 and 64 bit x86)
+* FreeBSD (amd64)
+
+Tier 2:
+
+* Mac OSX (> 10.10)
+* Android (arm/x86)
+* Apple IOS
+* Linux PPC64 (little endian)
+
+Tier 3:
+
+* Big Endian Linux
+* NetBSD
+* OpenBSD
+
+Unsupported (feel free to support this yourself)
+
+* AIX
+* zOS
+
+## For Windows
+
+To cross compile for windows on non windows platforms run:
+
+    $ make windows
+
+## For Other Linux
+
+To cross compile on linux for another archietecture:
+
+   # for rpi
+   $ make CROSS=ON TOOLCHAIN=contrib/armhf.toolchain.cmake
+
+   # for ppc64le
+   $ make CROSS=ON TOOLCHAIN=contrib/ppc64le.toolchain.cmake